<PAGE>

                                                                    EXHIBIT 10.3

                          PURCHASE AND SALE CONTRACT
                          --------------------------

     THIS PURCHASE AND SALE CONTRACT ("Contract") is entered into by and between
MACK-CALI TEXAS PROPERTY L.P., a Texas limited partnership ("Seller") and
PRENTISS PROPERTIES ACQUISITION PARTNERS, L.P., a Delaware limited partnership,
its permitted successors and assigns ("Purchaser").

                              W I T N E S S E T H:
                              --------------------

     FOR AND IN CONSIDERATION of the promises, undertakings, and mutual
covenants of the parties herein set forth, Seller hereby agrees to sell and
Purchaser hereby agrees to purchase and pay for all that certain property
hereinafter described in accordance with the following terms and conditions:

                                   ARTICLE I
                                   PROPERTY
                                   --------

     1.1  The conveyance by Seller to Purchaser shall include the tract of land
(the "Real Property") described in Exhibit A attached hereto, and made a part
                                   ---------
hereof for all purposes, together with all right, title, and interest of Seller,
if any, in and to any and all strips or gores, roads, easements, streets, and
ways bounding said tract, and all rights of ingress and egress thereto.

     1.2  The conveyance by Seller to Purchaser shall also include all
improvements and fixtures (other than fixtures owned by tenants) (the
"Improvements") of any kind whatsoever situated upon the Real Property,
including, but not limited to: the building known as "Cielo Center" and all
parking areas, and other improvements (other than improvements owned by tenants)
which are situated on the Real Property (the "Building").

     1.3  The conveyance by Seller to Purchaser shall also include all personal
property (except for such items as are owned by persons or entities renting
space in the Building as tenants or owned by licensees under licenses) of every
kind and character owned by Seller and now or hereafter installed, located,
situated, and used solely in or about or in connection with the operation, use,
and enjoyment of the Real Property and/or Improvements, including any
architectural plans and specifications, furniture, furnishings, and other
personal items, if any (the "Personal Property"). Personal Property shall not
include certain software and other related items which are proprietary to
Seller.

     1.4  All intangible property owned by Seller and used in connection with
each tract of the Real Property, the Personal Property or Improvements,
including but not limited to the Property Contracts (defined below) (all of
which intangible property together with all replacements or additions thereto
between the date hereof and the Closing (as hereinafter defined) is hereinafter
called the "Intangible Property").

     1.5  All of the Seller's interest in and to all leases, tenancy agreements,
concessions and other agreements (collectively, the "Tenant Leases") granting to
any party the right to use and occupy any portion of the Land or Improvements,
including all security and other deposits held by Seller thereunder.

     1.6  To the extent assignable, all dealer, manufacturer, builder or other
warranties pertaining in any way to the Improvements and Personal Property.

     1.7  All other assignable rights, privileges, appurtenances and licenses
owned by Seller and in any way related to the properties described above in this
Article 1.

                                  ARTICLE II
                                PURCHASE PRICE
                                --------------

     2.1  The purchase price to be paid by Purchaser to Seller for the Property
shall be the sum of Forty-Seven Million One Hundred Seventy-Five Thousand and
No/100 Dollars ($47,175,000.00). The purchase price shall be payable in cash or
other good funds to the Title Company (as hereinafter defined), and the Title
Company shall immediately deposit $25,000,000.00 (the "Escrowed Funds") into an
escrow account maintained by Chicago Title Insurance Company ("CTIC"), as escrow
agent, pursuant to an Escrow Agreement in the form of

                                       1
<PAGE>

Exhibit D attached hereto (the "Escrow Agreement"), and the Escrowed Funds shall
---------
be disbursed by CTIC from time to time in accordance with the terms and
provisions of the Escrow Agreement. The balance of the purchase price, plus or
minus adjustments, shall be disbursed by the Title Company to Seller at and as
part of the Closing.

                                  ARTICLE III
                                 EARNEST MONEY
                                 -------------

     3.1  Upon final execution of this Contract by all parties hereto, Purchaser
shall deliver the earnest money, by check, or wire transfer in the amount of
Fifty and No/100 Dollars ($50.00) ("Earnest Money") to First American Title
Insurance Company, 228 East 45th Street, New York, New York 10017-3303, Attn.:
Debra M. Hochman (the "Title Company").

     3.2  Contemporaneously with the execution of this Contract, Purchaser
hereby delivers to the Title Company, by check or wire transfer, in the amount
of Fifty and No/100 Dollars ($50.00) ("Independent Contract Consideration"),
which amount the parties bargained for and agreed to as consideration for
Seller's execution, delivery and performance of this Contract. This Independent
Contract Consideration is in addition to and independent of any other
consideration or payment provided in this Contract, is non-refundable, and shall
be retained by Seller notwithstanding any other provision of this Contract.

     3.3  In the event that this Contract is closed, then all Earnest Money
shall be disbursed to Seller and applied in partial satisfaction of the purchase
price. In the event that this Contract is not closed, then the Earnest Money
shall be disbursed in the manner provided for elsewhere herein. Notwithstanding
the foregoing sentence, if the Contract does not expressly provide that the
Earnest Money shall be disbursed to Seller, the Earnest Money shall be disbursed
to Purchaser.

                                  ARTICLE IV
                       PRE-CLOSING OBLIGATIONS OF SELLER
                       ---------------------------------

     4.1  Seller shall, at Seller's sole expense, within one (1) day from the
date hereof, furnish to Purchaser, or make available at the Property, the
following, to the extent the same are in the possession of Seller:

          (a)  An "As Built" Survey of the Real Property in the possession of
the Seller. Purchaser, at Purchaser's expense, may obtain an updated Survey of
the Real Property prior to the date of Closing, certified to the Purchaser and
the Title Company, such survey to be a Category 1A, Condition II survey as
prescribed by the Texas Land Surveyor's Association;

          (b)  A current commitment to issue an Owner's Policy of Title
Insurance with respect to the Real Property issued by First American Title
Insurance Company to Purchaser in the amount of $47,175,000.00, together with
good and legible copies of all documents constituting exceptions to Seller's
title as reflected in the Title Commitment;

          (c)  A list of all brokerage and listing agreements, construction
contracts (for tenant improvements or otherwise), space planning contracts,
service contracts, equipment leases, warranties, management, maintenance, or
other agreements ("Property Contracts") affecting the Property, together with
copies of such Property Contracts. Seller agrees not to enter into any
additional Property Contracts prior to Closing which would be binding on
Purchaser, unless such Property Contracts are approved in writing by Purchaser;

          (d)  Copies of all certificates of occupancy, permits, and other
governmental licenses or approvals;

          (e)  All site plans; construction drawings; environmental, mechanical,
electrical, structural, soils, roof, asbestos, and similar building inspection
reports; and "as built" plans and specifications relative to the Property in the
possession of Seller, if any;

          (f)  2000 operating budget for the Property;

          (g)  Historical income, expense and capital expenditure reports for
the last two (2) years;

                                       2
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          (h)  Copies of each Tenant Lease and all amendments and supplements
thereto; and (i) A current rent roll for the Property (the "Rent Roll"), which
Rent Roll is attached hereto as Exhibit E and made a part hereof.
                                ---------

Subject to the accuracy of the representation and warranty made by Seller in
Section 7.1(l), the Closing of the transaction described herein by Purchaser
shall be deemed satisfaction by Seller of the obligation to deliver documents
and materials pursuant to this Section 4.1 and Section 4.2 below.

     4.2  Seller shall promptly deliver to Purchaser such other documents as
Purchaser may reasonably request and shall further grant access to Purchaser to
all files on the Property at a location and a time to be mutually agreed upon by
the parties.

                                   ARTICLE V
                            SURVEY AND TITLE REVIEW
                            -----------------------

     5.1  Purchaser shall have a period until the date of Closing in which to
review and approve the Title Commitment and the Survey (the "Title Inspection
Period"). If the information to be provided pursuant to Sections 4.1(a) and (b)
reflects or discloses any defect, exception, or other matter affecting the
Property ("Title Defects") that is unacceptable to Purchaser, then prior to the
expiration of the Title Inspection Period, Purchaser shall provide Seller with
written notice of Purchaser's objections. Seller may, at its sole option, elect
to cure or remove the objections raised by Purchaser; provided, however, that
Seller shall have no obligation to do so, except Seller must remove each of the
following (the "Mandatory Cure Matters"): any deeds of trust securing borrowed
money against the Property and each conveyance of an interest in any of the
Property made after September 15, 2000 by Seller. Should Seller elect to attempt
to cure or remove the objections, Seller shall have ten (10) days from the date
of Purchaser's written notice of objections (the "Cure Period") in which to
accomplish the cure. In the event Seller either elects not to cure or remove the
objections or is unable to accomplish same prior to the expiration of the Cure
Period, then Purchaser shall be entitled, as Purchaser's sole and exclusive
remedy, either to terminate this Contract by providing written notice of
termination to Seller within five (5) days from the date of expiration of the
Cure Period or waive the objections and close the sale of the Property as
otherwise contemplated herein. The Title Inspection Period shall be deemed to be
extended through the Cure Period and the five (5) day period thereafter during
which time Purchaser has the right to terminate the Contract. Should Purchaser
terminate the Contract due to Title Defects during such five (5) day period,
Purchaser shall receive a refund of the Earnest Money. If the Purchaser shall
fail to notify Seller in writing of any objections to the state of Seller's
title to the Property as shown by the Title Commitment and the Survey to be
provided to Purchaser pursuant to Sections 4.1(a) and (b) hereof, then Purchaser
shall be deemed to have no objections to the state of Seller's title to the
Property as shown by the Title Commitment and the Survey, and any exceptions to
Seller's title which have not been objected to by Purchaser and which are shown
on the Survey or described in the Title Commitment shall be considered to be
"Permitted Exceptions". It is understood and agreed that the Property will be
conveyed to Purchaser subject to all Tenant Leases, and that such leases shall
be Permitted Exceptions. It is further understood and agreed that any Title
Defects which have been objected to by Purchaser and which are subsequently
waived by Purchaser shall be Permitted Exceptions.

                                  ARTICLE VI
                               INSPECTION PERIOD
                               -----------------

     6.1  Purchaser shall have until the date of Closing within which to inspect
the condition of the Property, to inspect those items listed in Sections 4.1(c)
through (h) ("Due Diligence Items") delivered to Purchaser, and to evaluate the
feasibility of purchasing the Property (the "Inspection Period"). Purchaser and
its duly authorized agents ("Purchaser's Agents") or representatives, upon prior
notice to Seller, shall be entitled to enter upon the Property at all reasonable
times during the Inspection Period in order to conduct engineering studies, soil
tests, environmental tests, asbestos tests, roof inspection, and any other
inspections and/or tests that Purchaser may deem necessary or advisable;
however, Purchaser may not conduct invasive testing without Seller's consent.
Seller may require that its representatives accompany Purchaser or Purchaser's
Agents in connection with the inspections to be conducted

                                       3
<PAGE>

pursuant to this Section 6.1; however, Seller covenants and agrees to make its
representatives available for such purpose; provided further, however, Purchaser
may conduct tenant interviews without the necessity of Seller's representatives
being present. Purchaser shall be responsible for any property damage caused to
Property or bodily injuries caused by Purchaser's Agents during the Inspection
Period. In the event that the inspections conducted pursuant to this Section 6.1
shows any condition to exist in the condition of the Property that is
unacceptable to Purchaser, in Purchaser's sole discretion, or if for any reason,
or for no reason whatsoever, Purchaser determines to not purchase the Property,
then Purchaser shall be entitled, as Purchaser's sole remedy, to cancel this
Contract by providing written notice of cancellation to Seller on or before the
expiration of the Inspection Period. If Purchaser shall provide written notice
of cancellation prior to the expiration of the Inspection Period, then this
Contract shall be cancelled, all Earnest Money shall be immediately returned to
Purchaser by the Title Company, and thereafter neither Seller nor Purchaser
shall have any continuing obligation one party unto the other party. On the
other hand, if Purchaser fails to provide written notice of cancellation to
Seller on or before the expiration of the Inspection Period, then the condition
of the Property shall be deemed to be acceptable to Purchaser, and Purchaser
shall no longer have the right to terminate this Contract pursuant to this
Article VI. Purchaser hereby indemnifies, protects, defends (with counsel
reasonably acceptable to Seller) and holds Seller and the Property free and
harmless from and against any and all costs, losses, liabilities, damages,
lawsuits, judgments, actions, proceedings, penalties, demands, attorneys' fees,
mechanic's liens, or expenses of any kind or nature whatsoever, arising out of
or resulting from (i) any entry and/or activities upon the Property by
Purchaser, Purchaser's agents, contractors and/or subcontractors, and/or the
contractors and subcontractors of such agents, or (ii) from the enforcement of
this agreement of indemnity or the assertion by Purchaser of any defense to its
obligations under this agreement of indemnity. The foregoing indemnity by
Purchaser shall survive the termination of this Contract or the Closing.

                                  ARTICLE VII
                 REPRESENTATIONS, WARRANTIES, AND COVENANTS OF
                 ---------------------------------------------
                             SELLER AND PURCHASER
                             --------------------

     7.1  Seller represents and warrants to Purchaser as follows:

          (a)  Seller will have at Closing good and indefeasible fee simple
title to the Real Property and the Improvements;

          (b)  There are no actions, suits, or proceedings pending or, to
Seller's knowledge, threatened against any portion of the Property, at law or in
equity, or before or by any federal, state, municipal, or other governmental
court, department, commission, board, bureau, agency, or instrumentality,
domestic or foreign;

          (c)  The execution by Seller of this Contract and the consummation by
Seller of the sale contemplated hereby have been duly authorized, and do not,
and, at the date of Closing, will not, to the best of Seller's knowledge, result
in a breach of any of the terms or provisions of, or constitute a default under
any indenture, agreement, instrument, or obligation to which Seller is a party
or by which the Property, or any portion thereof, is bound;

          (d)  Except as otherwise disclosed in writing to Purchaser by Seller
(including the matters described in Exhibit G hereto), Seller has not received
                                    ---------
any written notice of any violation of any ordinance, regulation, law, statute,
building code, zoning ordinance, or environmental laws pertaining to the
Property, or any portion thereof, which has not been cured or of any pending
zoning change or special assessment pertaining to the Property;

          (e)  Seller has not received any written notice of condemnation of the
Property;

          (f)  To Seller's knowledge and except as otherwise disclosed in
writing to Purchaser by Seller (including the matters described in Exhibit G
                                                                   ---------
hereto), all governmental approvals necessary for the operation of the Property
have been obtained and are in full force and effect, including all code
requirements regarding building occupancy and use, parking, and zoning;

          (g)  Except as disclosed in writing to Purchaser by Seller prior to
the date of Closing, in the Rent Roll or in Exhibit H hereto, (i) the Tenant
                                            ---------
Leases provided to Purchaser are

                                       4
<PAGE>

true and correct and are in full force and effect and have not been amended,
except pursuant to amendments delivered to Purchaser, (ii) the Rent Roll for the
Property provided by Seller to Purchaser is true and correct and is complete as
to the matters contained therein, (iii) Seller has not received or sent any
notices of default under any of the Leases that have not been cured, and Seller
has no knowledge of any material defaults under any of the Leases that have not
been cured, (iv) all rent due to Seller under Leases for the month of September,
2000 has been received by Seller, and (v) all security deposits paid under the
Leases are shown in the Rent Roll provided to Purchaser;

          (h)  For purposes hereof, Seller's knowledge shall mean the actual,
but not constructive, knowledge of Tim Jones, Daniel Wagner, Lani Abbott, Jim
Clabby, Jeff Kennemer and Barbara Crow, property manager for the Property,
Seller hereby representing to Purchaser that Jeff Kennemer, Barbara Crow and
Lani Abbott are the persons currently employed by Seller most likely to be aware
of such matters relating to the Property;

          (i)  There are no assignments for the benefit of creditors,
receiverships, conservatorships or voluntary or involuntary proceedings in
bankruptcy or pursuant to any other debtor relief laws contemplated by or filed
by Seller;

          (j)  Seller is not a "foreign person" as defined in the Internal
Revenue Code of 1986, as amended. If requested by Purchaser, Seller will deliver
to Purchaser at Closing a certificate so stating in a form complying with said
federal tax law;

          (k)  Seller has received no written notice that the Property or Seller
with regard to its ownership of the Property are currently subject to any
existing or threatened investigation or inquiry by any governmental authority or
to any remedial obligations under any applicable federal, state or local laws,
regulations or ordinances pertaining to health or the environment (hereinafter
sometimes collectively called "Applicable Environmental Laws"), including
without limitation the Comprehensive Environmental Response, Compensation and
Liability Act of 1980 ("CERCLA"), the Resource Conservation and Recovery Act of
1976 ("RCRA"), the Texas Water Code and the Texas Solid Waste Disposal Act; and

          (l)  The items described in Section 4.1(c), (g) and (h) that have been
either delivered to Purchaser or made available to Purchaser at the Property are
true, correct and complete in all material respects.

     7.2  Purchaser hereby represents and warrants to Seller as follows:

          (a)  Purchaser has full legal power and authority to enter into and
perform this Contract in accordance with its terms, and this Contract
constitutes the valid and binding obligation of Purchaser, enforceable in
accordance with its terms. The execution, delivery and performance of this
Contract and all documents in connection therewith are not in contravention of
or in conflict with any agreement or undertaking to which Purchaser is a party
or by which Purchaser may be bound or affected. The execution and delivery of
this Contract and the payment and performance by Purchaser of its payments and
obligations hereunder require no further action or approval in order to
constitute this Contract as a binding and enforceable obligation of Purchaser
and all such actions have been duly taken by Purchaser, or will be taken by
Purchaser prior to Closing; and

          (b)  All consents, approval, and authorizations from any person or
entity, and, to Purchaser's knowledge, any government or quasi-governmental body
or entity, required for the execution and delivery of this Contract by
Purchaser, the acquisition of the Property by Purchaser in accordance herewith
and the compliance by Purchaser with all of the provisions hereof, have been
obtained or prior to Closing will be obtained, and will be in effect as of the
date of the Closing.

          (c)  Seller's Environmental Inquiry. Purchaser acknowledges and agrees
               ------------------------------
that the sole inquiry and investigation Seller has conducted in connection with
the environmental condition of the Property is to obtain the certain
environmental report(s) described in Exhibit C attached hereto.
                                     ---------

          (d)  As Is. Purchaser is acquiring the Property "AS IS" without any
               -----
representation or warranty of Seller, express, implied or statutory, except for
the express

                                       5
<PAGE>

representations, warranties or certifications set forth in this Contract or the
implied warranties in the Deed, as to the nature or condition of or title to the
Property or its fitness for Purchaser's intended use of same. Except for the
express representations, warranties or certifications set forth in this
Contract, Purchaser is relying solely upon, and as of the Closing Date (as
hereinafter defined) will have conducted, its own, independent inspection,
investigation and analysis of the Property as it deems necessary or appropriate
in so acquiring the Property from Seller, including, without limitation, an
analysis of any and all matters concerning the condition of the Property and its
suitability for Purchaser's intended purposes, and a review of all applicable
laws, ordinances, rules and governmental regulations (including, but not limited
to, those relative to building, zoning and land use) affecting the development,
use, occupancy or enjoyment of the Property.

     Without limiting the generality of the foregoing but subject to the last
sentence of this paragraph, Purchaser hereby expressly waives and relinquishes
any actual or potential rights, legal claims, equitable claims and remedies
Purchaser may now or hereafter have against Seller, whether known or unknown,
with respect to any past, present or future presence or existence of "Hazardous
Materials" (as hereinafter defined) on, under or about the Property (or adjacent
or proximate to the Property) or with respect to any past, present or future
violations of any federal, state or local rules, regulations or laws or
liabilities (concerning, affecting or relating to the Property) which arise or
may arise under federal, state or local rules, regulations or laws (including
common law), now or hereafter enacted, regulating or governing the use,
handling, storage or disposal of Hazardous Materials, including, without
limitation, (i) any and all rights or claims Purchaser may now or hereafter have
to seek contribution from Seller under Section 113(f)(i) of the Comprehensive
Environmental Response Compensation and Liability Act of 1980 ("CERCLA"), as
amended by the Superfund Amendments and Reauthorization Act of 1986 (42 U.S.C.A.
(S)9613), as the same may be further amended or replaced by any similar law,
rule or regulation, (ii) any and all rights or claims Purchaser may have or
hereafter seek, whether known or unknown, now or hereafter existing, with
respect to the Property under Section 107 of CERCLA (42 U.S.C.A. (S)9607) and
(iii) any and all rights or claims Purchaser may now or hereafter have against
Seller under applicable Texas environmental laws concerning, affecting or
relating to the Property. As used herein, the term "Hazardous Material(s)"
includes, without limitation, any hazardous or toxic materials, substances or
wastes, such as (A) those materials identified or defined as such under federal,
state or local health or environmental laws, as amended from time to time, (B)
any materials, substances or wastes which are toxic, ignitable, corrosive or
reactive and which are regulated by any local governmental authority, any agency
of the State of Texas or any agency of the United States Government, (C)
asbestos, (D) petroleum and petroleum based products, (E) urea formaldehyde foam
insulation, (F) polychlorinated biphenyls (PCBs), and (G) freon and other
chloroflourocarbons. Notwithstanding anything in this Section 7.2(d), the
foregoing waiver and release shall not cover or include any violation of laws
arising out of or relating to Hazardous Materials first placed or suffered to
exist on the Property during Seller's ownership of the Property.

     The waivers and releases by Purchaser herein contained shall survive the
Closing and the recordation of the Deed and shall not be deemed merged into the
Deed upon its recordation.

     7.3 Seller covenants and agrees with Purchaser, from September 15, 2000
until 11:59 p.m. on September 21, 2000, as follows:

          (a)  Seller shall not sell, assign, or convey any right, title, or
interest whatsoever in or to the Property, or create or permit to exist any
mortgage, lien, security interest, easement, encumbrance, charge, or condition
affecting the Property (other than the Permitted Exceptions) without promptly
discharging the same prior to Closing; provided, the foregoing shall not include
liens suffered to exist by tenants under the Leases.

          (b)  Seller shall continue to (i) maintain the Property in its present
condition, ordinary wear and tear and damage described in Article XII hereof
excepted, and (ii) manage the Property in the same manner as it is currently
being managed;

          (c)  Seller shall cause to be maintained in force fire and extended
coverage insurance upon the Property, and public liability insurance with
respect to damage or injury to person or property occurring on the Property, in
at least such amounts as are maintained by Seller on the date hereof;

                                       6
<PAGE>

          (d)  Seller will not enter into any (i) new lease or any modification
of any existing lease at the Property, or (ii) any other contract with respect
to operation or maintenance of any portion of the Property; provided, Purchaser
has approved the leases and/or modifications described on Exhibit F hereto;
                                                          ---------

          (e)  Not, without the prior written consent of the Purchaser: (1)
permit any structural modifications or structural additions to the Property; or
(2) sell or permit to be sold or otherwise dispose of any item or group of items
constituting a portion of the Property other than obsolete items of Personal
Property which are replaced with new items of equal or better utility.

     7.4  All of the representations and warranties contained in Section 7.1 are
made by Seller both as of the date hereof and as of the date of Closing
hereunder. Notwithstanding anything to the contrary contained herein, it is
understood and agreed that the representations and warranties set forth in
Section 7.1 shall survive the Closing of this Contract for a period of one
hundred eighty (180) days following the date of Closing (the "Survival Period"),
and Seller shall have no liability of any kind whatsoever for any breach thereof
except to the extent a claim is asserted against Seller within the Survival
Period. Following the expiration of the Survival Period, the representations and
warranties contained in Section 7.1, to the extent a claim of a breach has not
been asserted by Purchaser during the Survival Period, shall be deemed to have
merged into the Deed. If any of the representations and warranties set forth
herein are determined by Seller or Purchaser at any time on or before the date
of Closing to be untrue or unfulfilled, then Purchaser, as its sole and
exclusive remedy, may terminate this Contract by providing written notice of
such termination to Seller, in which event the Earnest Money shall be returned
to Purchaser and thereafter neither Seller nor Purchaser shall have any further
liabilities or obligations one party unto the other party. The provisions of
this proposal shall expressly survive Closing.

     7.5  Each of the representations and warranties contained in Section 7.1
above and its various subparagraphs are intended for the benefit of Purchaser
and may be waived in whole or in part. All rights and remedies arising in
connection with the untruth or inaccuracy of any such representations and
warranties shall survive the Closing of the transaction contemplated hereby as
provided in Section 7.4 hereof, except to the extent that Seller gives Purchaser
written notice prior to Closing of the untruth or inaccuracy of any
representation or warranty, or Purchaser otherwise obtains actual knowledge
prior to Closing of the untruth or inaccuracy of any representation or warranty,
and Purchaser nevertheless elects to close this transaction. Purchaser shall be
deemed to have actual knowledge of the untruth or inaccuracy of any
representation or warranty only if (i) Purchaser receives written notice from
Seller satisfying the foregoing requirements, or (ii) either Dennis J. DuBois,
William J. Reister, Brad Park or Jason Berend has actual knowledge of any such
untruth or inaccuracy (Purchaser hereby representing to Seller that the
foregoing persons employed by or officers of Purchaser or its affiliate are the
persons most likely to be aware of whether Seller has breached any of its
representations and warranties); provided, Purchaser hereby represents that it
has made inquiry of Theresa Ledbetter, Don Bennett and Ralph Bistline, employees
of Purchaser or its affiliate(s), regarding the representations and warranties
of Seller in Section 7.1. Except to the extent otherwise expressly provided in
the immediately preceding sentence, no investigation, audit, inspection, review
or the like conducted by or on behalf of Purchaser shall be deemed to terminate
the effect of any such representations, warranties, covenants and
certifications, it being understood that Purchaser has the right to rely thereon
and that each such representation, warranty, covenant and certification
constitutes a material inducement to Purchaser to execute this Contract and to
close the transaction contemplated hereby and to pay the purchase price to
Seller. Notwithstanding anything herein to the contrary, Purchaser is deemed to
have actual knowledge of the information set forth in (a) the Tenant Leases and
Property Contracts delivered by Seller or made available by Seller at the
Property pursuant to Section 4.1(c) and (h) above, (b) the environmental reports
described on Exhibit C hereto, and (c) the reports and other materials
             ---------
transmitted under the letter dated September 18, 2000 from Martha Wach of Jones,
Day, Reavis & Pogue, which letter is attached hereto as Exhibit G.
                                                        ---------

     7.6  Purchaser covenants that it has or will offer employment to Barbara
Crow, Katherine Brinegar, Gabriel Hernandez, Trent Carter and Daniel Williams,
for at least the same salary and level of responsibility each such employee had
on the Closing Date for his or her respective employment in connection with the
management and operation of the Building. Purchaser is not assuming any
liabilities or obligation with respect to any such employee for any accrued
benefits or other compensation owed by Seller or its affiliates to any such
employee.

                                       7
<PAGE>

                                 ARTICLE VIII
                                    CLOSING
                                    -------

     8.1  The Closing (herein so called) shall take place by delivery of all
closing documents in escrow to the Title Company. The Closing shall occur on or
before September 21, 2000.

                                  ARTICLE IX
                        SELLER'S OBLIGATIONS AT CLOSING
                        -------------------------------

     9.1  At the Closing, Seller shall deliver to Purchaser the following:

          (a)  A special warranty deed covering the Property ("Deed"), duly
signed and acknowledged by Seller, which deed shall be in form for recording,
shall be mutually agreed upon by and between Purchaser and Seller.

          (b)  An Owner's Policy of Title Insurance (the "Title Policy") issued
by the Title Company on the standard form in use in the state where the Property
is located, with the costs of the Title Policy to be borne by Seller (except the
costs of the deletion of the survey exception pursuant to Section 9.1(b)(2)
hereof, which costs shall be borne by Purchaser should Purchaser request the
same), insuring good and indefeasible fee simple title to the Property in the
Purchaser, in the amount of $47,175,000.00, containing the exceptions approved
by Purchaser at the Closing.

          (c)  A bill of sale, duly executed and acknowledged by Seller,
conveying to Purchaser the Personal Property, if any, with covenants of special
warranty, subject only to the Permitted Exceptions.

          (d)  Executed originals of all of the Tenant Leases (to the extent
available, and if not available copies thereof certified by Seller), and all
amendments and modifications thereto, covering space in the Building then in
effect.

          (e)  An assignment and assumption agreement, duly executed and
acknowledged by Seller, assigning all of Seller's interest in and to such leases
to Purchaser.

          (f)  A letter to the tenants, in a form approved by Purchaser,
notifying the tenants of the change of ownership and directing tenants to pay
all future rent to Purchaser.

          (g)  An assignment and assumption agreement, duly executed and
acknowledged by Seller, assigning all of its rights, if assignable, under all of
the Property Contracts other than (i) a contract with Apex Site Management, and
(ii) all management and leasing contracts, which contracts described in clauses
(i) and (ii) are to be terminated on or before the Closing Date by Seller at its
sole cost and expense. The provisions of this Section 9.1(g) shall survive the
Closing.

          (h)  An assignment and assumption agreement, duly executed and
acknowledged by Seller, assigning all of Seller's right, title and interest in
and to the Intangible Property.

          (i)  Such evidence or other documents that may be reasonably required
by Purchaser and the Title Company evidencing the status and capacity of Seller
and the authority of the person or persons who are executing the various
documents on behalf of Seller in connection with the sale of the Property.
Seller shall also deliver to Purchaser any other document or instrument
reasonably requested by Purchaser in connection with the sale of the Property.

          (j)  A non-withholding statement that will satisfy the requirements of
Section 1445 of the Internal Revenue Code so that Purchaser is not required to
withhold any portion of the purchase price for payment to the Internal Revenue
Service.

          (k)  All keys to the Property, all tenant records (provided Seller, at
its sole cost and expense, shall have access to such records and the right to
make copies of the same for a period of three (3) years from the Closing),
leasing brochures and related materials and all telephone exchanges.

                                       8
<PAGE>

          (l)  All original plans, drawings, specifications, architectural
documents, building permits, maintenance records, certificates of occupancy and
governmental licenses for improvements pertaining to the Property in the
possession of Seller;

          (m)  The original of all Property Contracts or copies, certified by
Seller, if originals are not available;

          (n)  Evidence that all management and leasing contracts affecting any
portion of the property have been terminated; and

          (o)  Mack-Cali Realty, L.P. and Mack-Cali Realty Corporation
(collectively, "Mack Entities") shall execute and deliver to Purchaser a
termination and release agreement (the "Termination and Release Agreement") in
the form of Exhibit B hereto terminating the obligations of Purchaser, Prentiss
            ---------
Properties Trust ("PPT") and the Mack Entities under that certain Agreement and
Plan of Merger dated June 27, 2000 and executed by all such parties.

Any certificate of Seller delivered pursuant to Section 9.1(d) or (m) above that
certifies to the authenticity of Tenant Leases or Property Contracts executed by
any owner of the Property that preceded Seller shall be subject to a one hundred
eighty (180) day survival period.

                                   ARTICLE X
                       PURCHASER'S OBLIGATIONS AT CLOSING
                       ----------------------------------

     10.1  At Closing, Purchaser shall deliver to Seller the following:

          (a)  The purchase price, in cash.

          (b)  The assignment and assumption agreements described in Sections
9.2(e), (g) and (h) above executed by Purchaser.

          (c)  Such evidence or other documents that may be reasonably required
by Seller and the Title Company evidencing the status and capacity of Purchaser
and the authority of the person or persons who are executing the various
documents on behalf of Purchaser in connection with the sale of the Property.

          (d)  Purchaser and PPT shall execute and deliver to Seller the
Termination and Release Agreement.

          (e)  Purchaser shall also deliver to Seller any other document or
instrument reasonably requested by Seller in connection with the sale of the
Property.

          (f)  Copies of all the notices Purchaser is sending to each tenant
under the Tenant Leases advising each such tenant, among other things, that
Purchaser has received each such tenant's security deposits under the Tenant
Leases.

                                  ARTICLE XI
                     COSTS AND ADJUSTMENTS AND INDEMNITIES
                     -------------------------------------

     11.1  Seller shall pay for the costs of the Title Policy (except the survey
deletion costs), and the recording of the special warranty deed and one-half (
1/2) of the escrow fees, if any.  Purchaser shall pay the costs of the survey
deletion (should Purchaser request the same), any tax stamps or other state or
local charges incurred in connection with such transfer, and one-half ( 1/2) of
the escrow fees, if any.  Seller and Purchaser shall be responsible for the fees
and expenses of their respective attorneys.  All other costs shall be
apportioned between Purchaser and Seller in accordance with customary standards
for the state and county where the Property is located.

     11.2  At Closing, the following items shall be adjusted or prorated between
Seller and Purchaser, with the date of Closing to be credited to Purchaser:

          (a)  All ordinary real property taxes levied or assessed against the
Property by the city, county, or state shall be prorated between Purchaser and
Seller on the basis of the latest available tax assessments. The apportionment
of taxes shall be upon the basis of the tax rate for the last preceding year (if
the current year's statements are not available) applied to the latest assessed
valuation, and adjustments in the prorations shall be made if necessary upon
receipt of

                                       9
<PAGE>

the tax statements for the year of Closing, and both parties agree that payment
of the amount of such adjustments shall be made within ninety (90) days of
receipt of such tax statements for the year of Closing. If Purchaser receives
after the Closing Date any refund of ad valorem taxes paid for the Property for
years prior to the calendar year commencing January 1, 2000, Purchaser shall pay
any such refund to Seller. The provisions of this paragraph shall expressly
survive Closing.

          (b)  Rents payable with respect to the Property for the then current
month shall be prorated effective as of the date of Closing; provided, however,
no proration shall be made for delinquent rents existing as of the date of
Closing. With respect to such delinquent rents, Purchaser shall make a
reasonable attempt to collect the same for Seller's benefit after Closing in the
usual course of operation of the Property, and any such collection shall be
remitted to Seller promptly upon receipt by Purchaser; provided, however, that
nothing contained herein shall operate to require Purchaser to institute any
lawsuit or other collection procedure to collect such delinquent rents. Seller
may institute collection proceedings against any such delinquent tenants, but
Seller agrees not to attempt to evict any such tenant. In this regard, the first
monies collected from tenants owing delinquent rent shall be applied to current
rents and retained by Purchaser; with any overage to be forthwith paid by
Purchaser to Seller. At the Closing, Seller shall pay to Purchaser, in cash, the
amount of any rents prepaid to Seller by tenants of the Property for periods
subsequent to the Closing Date. Attached hereto as Exhibit H is a schedule of
                                                   ---------
delinquencies and other amounts due and owing to Seller under the Tenant Leases
as of the Closing Date.

          (c)  Seller shall pay to Purchaser, as a credit against the purchase
price, the amount of any security deposits held by Seller pursuant to any Tenant
Leases and shall provide Purchaser with any letters of credit securing the
performance of tenants under their leases. Seller shall execute such documents
as may be reasonably requested by Purchaser and required by the issuers of any
such letters of credit in order to assign such letters of credit to Purchaser,
which agreement by Seller shall survive the Closing.

          (d)  All other income and ordinary operating expenses for or
pertaining to the Property, including, but not limited to, public utility
charges, maintenance, property and association fees, service charges and all
other normal operating charges of the Property shall be prorated as of the date
of Closing. Seller shall be solely responsible for all leasing commissions and
landlord construction obligations under all Tenant Leases and expansions,
renewals or extensions thereof executed on or prior to the date of Closing other
than those described on Exhibit I hereto which are to be paid by Purchaser, and
                        ---------
the provisions of this sentence shall survive Closing.

          (e)  If the actual amount of the income and operating expenses and
other items adjusted at Closing in connection with the Property are not
available as of the Closing, such items shall be prorated on the basis of the
best information then available to Purchaser and Seller, with a subsequent cash
adjustment, if necessary, of the proration to be made between Purchaser and
Seller when the actual amounts of such items are available. The provisions of
this paragraph shall expressly survive Closing.

          (f)  In the event any adjustments pursuant to this Section 11.2 are
determined to be erroneous, then either party hereto who is entitled to
additional monies shall invoice the other party for such additional amounts as
may be owing, and such amounts shall be paid within thirty (30) days from the
receipt of any such invoice. Nothing contained in this subsection 11.2(f) shall
prevent either party from disputing any claim made by the other party that an
adjustment made at Closing was erroneous. The provisions of this paragraph shall
expressly survive Closing.

          (g)  Purchaser acknowledges that (i) Partners Holdings, Inc. ("PHI")
is being billed for certain tenant improvements performed by Seller for which
PHI is responsible (the "PHI TI") and (ii) the rent bills for October 2000 which
are being sent include bills for certain tenant extras (such as HVAC charges,
security cards and computer room charges) (the "Tenant Charges") through
September 10, 2000. Purchaser agrees to remit to Seller the payment for the PHI
TI and the Tenant Charges upon receipt of same. In addition, Purchaser agrees to
bill the tenants, for Seller's account, for the Tenant Charges for the period
from September 11, 2000 through the Closing Date.

                                       10
<PAGE>

     11.3  Seller agrees to indemnify and hold Purchaser harmless of and from
any and all liabilities, claims, demands, and expenses, of any kind or nature
(except those items which under the terms of this Contract specifically become
the obligation of Purchaser) due to acts, omissions or occurrences prior to the
date of Closing and which are in any way related to the ownership, maintenance,
or operation of the Property, and all expenses related thereto, including, but
not limited to, court costs and attorneys' fees.

     11.4  Purchaser agrees to indemnify and hold Seller harmless of and from
any and all liabilities, claims, demands, and expenses, of any kind or nature
(except those items which under the terms of this Contract specifically are the
obligations of Seller) due to acts, omissions or occurrences, on or subsequent
to the date of Closing and which are in any way related to the ownership,
maintenance, or operation of the Property, and all expenses related thereto,
including, but not limited to, court costs and attorneys' fees.

The provisions of Sections 11.3 and 11.4 shall expressly survive Closing.

                                  ARTICLE XII
       DAMAGE OR DESTRUCTION OR TAKING OF PRIOR TO CLOSING; CONDEMNATION
       -----------------------------------------------------------------

     12.1  Notwithstanding any provisions of the Texas Property Code, in the
event that the Improvements or any of the items constituting the Personal
Property should be damaged by any casualty prior to Closing, then if the cost of
repairing such damage, as estimated by an architect or contractor retained
pursuant to the mutual agreement of Seller and Purchaser, is:

          (a)  less than One Hundred Thousand and No/100 Dollars ($100,000.00),
then the Seller shall repair such damage as promptly as is reasonably possible,
restoring the damaged property at least to its condition immediately prior to
such damage; and, in the event such repairs have not been completed prior to
Closing, then the Closing shall nevertheless proceed as scheduled, Seller shall
assign to Purchaser all insurance proceeds payable for such damage and Purchaser
shall receive a credit against the purchase price for the amount of any
deductible provided in the applicable insurance policy;

          (b)  greater than One Hundred Thousand and No/100 Dollars
($100,000.00), then, at Purchaser's election, Seller shall pay to Purchaser, at
Closing, all insurance proceeds payable for such damage, and the sale shall be
closed without Seller's repairing such damage but with Purchaser receiving a
credit for the amount of any deductible provided for in the applicable insurance
policy, or, if Purchaser does not elect to accept such insurance proceeds, then
Purchaser may elect to terminate this Contract, in which case the Earnest Money
shall be refunded to Purchaser and neither party shall have any further rights,
duties or obligations one to the other hereunder.

     12.2  In the event that there is a taking of all or any portion of the Real
Property and/or Improvements prior to Closing or Seller receives a threat
thereof, evidenced by, written notice from a governmental authority having
condemnation power then Purchaser may elect to either terminate this Contract
(in which event the Earnest Money shall be refunded to Purchaser), or close and
receive all condemnation proceeds. Seller agrees not to settle or compromise any
award thereof without Purchaser's consent, in the event Purchaser elects to
close and receive the condemnation proceeds.

                                 ARTICLE XIII
                             POSSESSION OF PROPERTY
                             ----------------------

     13.1  Possession of the Property free and clear of the rights of all
parties, except parties claiming the Permitted Exceptions and the Tenant Leases
and parties under the Property Contracts, shall be delivered to Purchaser at
Closing.

                                  ARTICLE XIV
                                    NOTICES
                                    -------

     14.1  All notices, demands, or other communications of any type given by
the Seller to the Purchaser, or by the Purchaser to the Seller, whether required
by this Contract or in any way related to the transaction contracted for herein,
shall be void and of no effect unless given in accordance with the provisions of
this paragraph. All notices shall be in writing and delivered to the person to
whom the notice is directed, either by fax, in person or by United States Mail,
as a

                                       11
<PAGE>

registered or certified item, return receipt requested, or by Federal Express or
other reputable overnight courier. Notices delivered by fax shall be deemed
received upon sender's receipt of an electronic confirmation that such
transmission was successful on or before 5:00 p.m. (New York time). Notices
delivered by mail shall be deemed given two (2) days after deposited in a post
office or other depository under the care or custody of the United States Postal
Service, enclosed in a wrapper with proper postage affixed, addressed as
follows:

     Purchaser:          Prentiss Properties Acquisition Partners, L.P.
                         3860 W. Northwest Highway, Suite 400
                         Dallas, Texas  75220
                         Attn: Dennis J. DuBois
                         Fax No.:  (214) 358-7122

     With a copy to:     Prentiss Properties Acquisition Partners, L.P.
                         3890 W. Northwest Highway, Suite 400
                         Dallas, Texas  75220
                         Attn: J Kevan Dilbeck
                         Fax No.: (214) 350-2409

     Seller:             Mack-Cali Texas Property L.P.
                         11 Commerce Drive
                         Cranford, New Jersey  07016
                         Attn.: Mitchell E. Hersh, Chief Executive Officer, and
                                Roger W. Thomas, Executive Vice President,
                                General Counsel and Secretary

     With a copy to:     Pryor Cashman Sherman & Flynn, L.L.P.
                         410 Park Avenue
                         New York, New York  10022
                         Attn.: Andrew S. Levine, Esq.

Notice given by hand delivery shall also be permitted and shall be deemed given
and received upon delivery thereof to the address of the proposed recipient at
the address set forth above. Notices by overnight courier shall be effective one
(1) business day following deposit with such courier. Regardless of the method
of delivery, notice actually received by the party to whom addressed shall
always be effective upon actual receipt.

                                  ARTICLE XV
                                   REMEDIES
                                   --------

     15.1 In the event all conditions of this Contract are satisfied and in the
event all covenants and agreements to be performed prior to Closing are fully
performed, and in the event that performance of this Contract is tendered by the
Purchaser and the sale is not consummated through a default on the part of
Seller on the date of Closing, then Purchaser, as its sole remedy, shall be
entitled to cancel this Contract by providing written notice thereof to Seller
on or before September 21, 2000, in which event the Earnest Money shall be
returned immediately to Purchaser by the Title Company and the parties hereto
shall have no further liabilities or obligations one party unto the other party.
Except as set forth in this Article XV, in no event shall Purchaser have the
right to sue Seller for damages, bring any action against the Property, or file
notice of lis pendens regarding any claim of Purchaser as a result of any
default by Seller prior to the date of Closing.

     15.2 In the event that Purchaser does not cancel this Contract prior to the
expiration of the Inspection Period and fails to close the acquisition of the
Property through no fault of Seller or because of any other right granted to
Purchaser under this Contract to terminate the same; then such failure shall be
an event of default hereunder, and Seller's sole remedy, as Seller's total
damages and relief, shall be to receive the Earnest Money and terminate this
Contract. The Earnest Money is agreed upon by and between the Seller and
Purchaser as liquidated damages due to the difficulty and inconvenience of
ascertaining and measuring actual damages, and the uncertainty thereof. No other
damages, rights, or remedies shall in any case be collectible, enforceable, or
available to the Seller other than as set forth in this paragraph defined.
Seller shall have no right to seek consequential or punitive damages from
Purchaser.

                                       12
<PAGE>

                                  ARTICLE XVI
                                  ASSIGNMENT
                                  ----------

     16.1 Purchaser shall have the right to assign this Contract to any person,
firm, corporation, or other entity controlling, controlled by or under common
control with Purchaser or in which Purchaser or any of Purchaser's principals
(or their affiliates) hold a controlling interest. From and after such
nomination or assignment, wherever in this Contract reference is made to
Purchaser such reference shall mean the nominee or assignee who shall succeed to
all the rights, duties, and obligations of Purchaser hereunder.

                                 ARTICLE XVII
                                INTERPRETATION
                                --------------

     17.1 Where required for proper interpretation, words in the singular shall
include the plural; the masculine gender shall include the neuter and the
feminine, and vice versa.  The terms "successors and assigns" shall include the
heirs, administrators, executors, successors, and permitted assigns, as
applicable, of any party hereto.

                                 ARTICLE XVIII
                                   AMENDMENT
                                   ---------

     18.1 This Contract may not be modified or amended, except by an agreement
in writing signed by the Seller and the Purchaser. The parties may waive any of
the conditions contained herein or any of the obligations of the other party
hereunder, but any such waiver shall be effective only if in writing and signed
by the party waiving such conditions and obligations.

                                  ARTICLE XIX
                                ATTORNEYS' FEES
                                ---------------

     19.1 In the event it becomes necessary for either party to file a suit to
enforce this Contract, or any provisions contained herein, the prevailing party
shall be entitled to recover, in addition to all other remedies or damages,
reasonable attorneys' fees and costs of court incurred in such suit.

                                  ARTICLE XX
                             DESCRIPTIVE HEADINGS
                             --------------------

     20.1 The descriptive headings of the several paragraphs contained in this
Contract are inserted for convenience only and shall not control or affect the
meaning or construction of any of the provisions hereof.

                                  ARTICLE XXI
                               ENTIRE AGREEMENT
                               ----------------

     21.1 This Contract, and the items to be furnished in accordance herewith,
constitutes the entire agreement between the parties pertaining to the sale of
the Property and supersedes all prior and contemporaneous agreements and
understandings of the parties in connection with the sale of the Property.  No
representation, warranty, covenant, agreement, or condition not expressed in
this Contract shall be binding upon the parties hereto or shall affect or be
effective to interpret, change, or restrict the provisions of this Contract.

                                 ARTICLE XXII
                            MULTIPLE ORIGINALS ONLY
                            -----------------------

     22.1 Numerous copies of this Contract may be executed by the parties
hereto. Each such executed copy shall have the full force and effect of an
original executed instrument.

                                 ARTICLE XXIII
                                  ACCEPTANCE
                                  ----------

     23.1 Time is of the essence of this Contract.  The effective date of this
Contract by Seller shall be the date of execution of this Contract by the last
person to execute on behalf of Seller or Purchaser.  If the final date of any
period referred to herein or any specific date for performance of an obligation
or the exercise of a right falls upon a Saturday, Sunday, or legal

                                       13
<PAGE>

holiday under the laws of the state where the Property is located, then in such
event the time of such period shall be extended to the next day which is not a
Saturday, Sunday, or legal holiday under the laws of the state where the
Property is located.

                                 ARTICLE XXIV
                            REAL ESTATE COMMISSION
                            ----------------------

     24.1 Seller represents and warrants to Purchaser that Seller has not
contacted or entered into any agreement with any real estate broker, agent,
finder, or any other party in connection with this transaction other than
Donaldson, Lufkin, Jenrette or an affiliate thereof ("DLJ"), and that Seller has
not taken any action which would result in any other real estate broker's,
finder's, or other fees or commissions being due and payable to any other party
with respect to the transaction contemplated hereby. Seller shall be responsible
for all commissions payable by DLJ.

     24.2 Purchaser hereby represents and warrants to Seller that Purchaser has
not contracted or entered into any agreement with any real estate broker, agent,
finder, or any other party in connection with this transaction, and that
Purchaser has not taken any action which would result in any real estate
broker's, finder's, or other fees or commissions being due or payable to any
party with respect to the transaction contemplated hereby.

     24.3 Each party hereby indemnifies and agrees to hold the other party
harmless from any loss, liability, damage, cost, or expense (including
reasonable attorneys' fees) resulting to the other party by reason of a breach
of the representation and warranty made by such party herein. Notwithstanding
anything to the contrary contained herein, the indemnities set forth in this
Article XXIV shall survive the Closing.

                                  ARTICLE XXV
                             INTENTIONALLY DELETED
                             ---------------------

                                 ARTICLE XXVI
                            LIMITATION OF LIABILITY
                            -----------------------

     26.1 Notwithstanding anything herein to the contrary, any and all
liabilities of Seller and Purchaser shall be satisfied solely out of the
properties and other assets of Seller and Purchaser, as the case may be, which
may exist from time to time. Any properties and assets of the partners in Seller
or Purchaser, or of the venturer's, partner's, employees, directors, officers,
principals and shareholders of such partners, shall not be subject to the
satisfaction of any such liabilities and obligations of Seller or Purchaser, as
the case may be, under this Contract. The Closing documents to be executed by
Seller and Purchaser at Closing may, at each such party's option, contain the
foregoing limitation in Seller's and Purchaser's liability.

                                 ARTICLE XXVII
                               LIKE-KIND EXCHANGE
                               ------------------

     27.1 Seller and Purchaser agree to cooperate reasonably with the other
party hereto in effecting an exchange transaction which includes the Property,
pursuant to Section 1031 of the United States Internal Revenue Code, provided
that any such exchange transaction, and the related documentation, shall: (a) be
at the sole cost and expense of the party desiring to effect such exchange
transaction; (b) not require the party hereto which has been requested to
cooperate in an exchange transaction to execute any contract, make any
commitment, or incur any obligations, contingent or otherwise, to third parties;
(c) not cause the party hereto which has been requested to cooperate in an
exchange transaction to be liable or potentially liable for any environmental
conditions affecting property other than the Property; (d) not delay the
Closing; (e) not include the party hereto which has been requested to cooperate
in an exchange transaction acquiring title to any property other than the
Property or otherwise becoming involved in a transaction with a third party; and
(f) not otherwise be contrary to or inconsistent with the terms of this
Contract.

                          [SIGNATURE PAGE TO FOLLOW]

                                       14
<PAGE>

RECEIPT OF EARNEST MONEY AND ONE (1) EXECUTED COUNTERPART OF THIS CONTRACT IS
HEREBY ACKNOWLEDGED THIS 21ST DAY OF SEPTEMBER, 2000.

SELLER:
------

MACK-CALI TEXAS PROPERTY L.P.,
a Texas limited partnership

By:  Mack-Cali Sub XVII, Inc., general partner

          /s/ JAMES A. CLABBY
     By:____________________________
              James A. Clabby
     Name:__________________________
              Senior Vice President
     Title:_________________________


EXECUTED on this the 21st day of September, 2000.


PURCHASER:
---------

PRENTISS PROPERTIES ACQUISITION PARTNERS, L.P.,
a Delaware limited partnership

By:  Prentiss Properties I, Inc., general partner


           /s/ J. KEVAN DILBECK
     By: ___________________________________
               J. Kevan Dilbeck
     Name:__________________________________
               Senior Vice President
     Title:_________________________________


EXECUTED on this the 21st day of September, 2000.

                                       15