<PAGE>

                                                                   EXHIBIT 10.15

                              FIRST AMENDMENT TO

                AGREEMENT FOR THE PURCHASE AND SALE OF PROPERTY

                      FOR THE AT&T CALL CENTER BUILDINGS
<PAGE>

               FIRST AMENDMENT TO REAL ESTATE PURCHASE CONTRACT
               ------------------------------------------------

     THIS FIRST AMENDMENT TO REAL ESTATE PURCHASE CONTRACT is executed by the
undersigned parties to that certain "Agreement for the Purchase and Sale of
Property" dated June 22, 2000 (the "Agreement"), providing for the purchase and
sale of certain real property located in Oklahoma County, Oklahoma (the
"Property").  The Agreement is amended in the following respects:

     1.   By agreement of the parties, the date of the Closing shall be December
28, 2000.  This First Amendment shall be effective as of the date of the Closing
(the "Effective Date").

     2.   Purchaser hereby assigns all of its rights and duties under the
Agreement to Wells Fund XII-REIT Joint Venture Partnership, a Georgia joint
venture partnership ("Assignee").  Assignee is an affiliate of Purchaser.

     3.   Seller hereby represents and warrants to Assignee that all
representations and warranties set forth in Section 7 of the Agreement are true
and correct as of the Effective Date.  In order to give Assignee additional
comfort relative to the representation and warranty contained in Section 7(t) of
the Agreement, Seller also agrees to use commercially reasonable efforts to
obtain and deliver to Assignee an estoppel certificate in substantially the form
attached hereto as Exhibit "A" as soon as reasonably possible after the
Effective Date.

     4.   As of the Effective Date, as evidenced by the acceptance letters
attached hereto as Exhibit "B", AT&T Corp. has accepted all of the Existing
Space (as defined in the Lease with AT&T Corp.)  and all of the Expansion Space
(as defined in the Lease with AT&T Corp.). Seller represents and warrants to
Assignee that AT&T Corp. is presently occupying all of the referenced space
except for certain portions of the Expansion Space located on the first floor of
the building (the "First Floor Space").  The Seller specifically agrees that,
notwithstanding the Closing, it will remain responsible for all of the following
matters (collectively, the "Post-Closing Obligations"):

          (a)  the delivery to Purchaser of an estoppel certificate executed by
     AT&T Corp. in substantially the form attached as Exhibit "D" to the
     Agreement;

          (b)  the exterior punchlist items described in Section 5 of the Tenant
     Estoppel Certificate executed by Jordan Associates, Inc.;


                                       1
<PAGE>

          (c)  the satisfaction of any requirements necessary to cause the
     temporary certificate of occupancy issued for the Expansion Space to become
     a permanent certificate of occupancy; and

          (d)  issuance by Smith-Roberts Associates, Inc. of a revision of its
     ALTA/CSM Land Title Survey, dated December 22, 2000, to delete therefrom
     the reference in the "Notes" to an "Unrecorded Easement", to reflect
     thereon the release of that portion of the water easement in favor of the
     City of Oklahoma City and that portion of the electricity easement in favor
     of Oklahoma Gas and Electric Company shown as running beneath a portion of
     the building.

     Seller shall resolve all of the referenced items at its expense as soon as
reasonably possible after the Effective Date.  If delays in the Seller's final
completion of the Post-Closing Obligations result in rental loss or other
damages to Assignee, Seller (and TCC pursuant to Section 5 below), will
indemnify and hold Assignee harmless with respect to any such damages.  Assignee
will cooperate with Seller as reasonably necessary to allow Seller to resolve
the referenced items and to recover from AT&T Corp. the rent for the Expansion
Space for the month of January 2001, any sums owed to Seller for rent and/or
operating cost reimbursements for the period prior to closing, and any sums owed
in connection with tenant improvement issues.

     5.   Seller agrees to cause Trammell Crow Company ("TCC") to issue a letter
of  guaranty, in the form attached hereto as Exhibit "C", whereby TCC will
guarantee the Seller's performance of the Post-Closing Obligations.

     6.   Seller agrees to instruct the Title Agent to withhold from its sales
proceeds and place in escrow the sum of $100,000 to further ensure the Seller's
performance of the Post-Closing Obligations.  At such time as Seller has
performed the Post-Closing Obligations, Purchaser will confirm the same in a
letter directed to the Title Agent, and the Title Agent will promptly release
all such funds to Seller; provided, however, in the event Seller fails to
perform the Post-Closing Obligations within 120 days after the Effective Date,
Assignee shall be entitled to receive an amount equal to $10,000.00 per month
from the referenced escrow for each month beyond such 120 day period that such
non-performance continues, until all Post-Closing Obligations are completed.

     7.   Seller agrees to instruct the Title Agent to pay to Assignee from
Seller's proceeds January rent attributable to the Expansion Space in the amount
of $78,500.00.  In the event Assignee receives a payment from AT&T Corp. for
some or all of the January rent attributable to the Expansion Space, Assignee
shall remit the same to Seller.

     Except as otherwise provided herein, all of the terms and provisions of the
Agreement shall remain in full force and effect.  Capitalized terms which are
not defined in this First Amendment shall have the meanings specified in the
Agreement.


                                       2
<PAGE>

     IN WITNESS WHEREOF, the parties have executed this First Amendment with the
intent that it be effective as of the Effective Date.



SELLER:                       OKC REAL ESTATE INVESTMENTS, INC., a Delaware
                              corporation

                                     /s/ Shaun Frankfurt
                              By:------------------------------------------
                                 Name:   Shaun Frankfurt
                                 Title:  President



PURCHASER:                    WELLS CAPITAL, INC.

                                     /s/ Douglas P. Williams
                              By:------------------------------------------
                                         Douglas P. Williams
                                 Name:-------------------------------------
                                 Title:  Executive V.P.
                                       ------------------------------------


                                       3
<PAGE>

APPROVED AND AGREED:
-------------------


WELLS FUND XII-REIT JOINT
VENTURE PARTNERSHIP, a Georgia joint venture partnership

By:  Wells Operating Partnership, L.P.,
     a Delaware limited partnership, as
     Administrative Venture Partner

By:  Wells Real Estate Investment
     Trust, Inc., a Maryland corporation,
     as General Partner



By: /s/ Douglas P. Williams
   --------------------------------------
   Name:  Douglas P. Williams
        ---------------------------------
   Title: Executive V.P
         --------------------------------


                                       4
<PAGE>

ACKNOWLEDGMENT BY ESCROW AGENT:
------------------------------

     The undersigned, as the Escrow Agent under the Agreement, agrees to comply
with the escrow provisions contained in the foregoing First Amendment to Real
Estate Purchase Contract, subject to the terms and conditions of Section 30 of
the Agreement.


                                              FIRST AMERICAN TITLE & TRUST
                                              COMPANY, Agent for First American
                                              Title Insurance Company


                                              By: /s/ Betty J. Cummins
                                                 -------------------------------
                                                 Name:  Betty J. Cummins
                                                 Title: Vice President

                                       5
<PAGE>

                                  EXHIBIT "A"
                                  -----------

                                   ESTOPPEL


Wells Fund XII-REIT Joint Venture Partnership
c/o Wells Management Company, Inc.
6200 The corners Parkway, Suite 250
Norcross, GA 30092

     Re:  Declaration of Protective Covenants, Quail Springs Office Park, dated
          April 27, 1983, and recorded in Book 4997, page 999, as amended by
          instruments recorded in Book 5230, page 44 and Book 7269, page 735
          (collectively, the "Declaration")

Ladies and Gentlemen:

     The undersigned is the Association under the Declaration and, understanding
that you have entered into a contract to purchase the real property and
improvements thereon (collectively, the "Property"), commonly known as 3201
Quail Springs Parkway, and further understanding that you  and any lender and/or
successor owner of the Property (collectively, "Owner"), will rely on the
information herein contained, hereby certifies to you as follows:

     1.   The Property is subject to the provisions of the Declaration and is
entitled to the rights, privileges and benefits thereunder.

     2.   The charges owed by the Owner (or any predecessor) to the Association
for the calendar year 2000 have been paid in full. The Annual Budget for the
Year 2001 is estimated to be $__________, is due and payable on or before
______________, 2001, and the portion thereof applicable to the Property is
$__________. There are no outstanding special assessments for capital
improvements and none are anticipated in the foreseeable future. There are no
other charges, expenses or other costs which are due and payable by Owner.

     3.   That to the best of the undersigned's knowledge, the Declaration is in
full force and effect, the Property, as improved, and the Owner are in full
compliance with the terms and conditions of the Declaration, including all plan
approval requirements, no event of default or default has occurred and is
continuing under the Declaration with respect to the Property and no event has
occurred which, with the passage of time or the giving of notice, or both, would
constitute an event of default or default.

     4.   The Association is a duly organized and validly formed nonprofit
corporation under the laws of the State of Oklahoma, is in good standing in the
State of Oklahoma, is not subject to any involuntary proceeding for dissolution
or liquidation thereof, and has obtained all requisite authorizations to execute
this Estoppel Certificate.
<PAGE>

     5.   The Declarant under the Declaration continues to be _________________.

     Agreed to this ______ day of December, 2000.

                                    Association:
                                    Quail Springs Office Park Owners'
                                    Association, Inc., an Oklahoma nonprofit
                                    corporation


                                    By:_________________________________________
                                       Name:____________________________________
                                       Title:___________________________________
<PAGE>

                                  EXHIBIT "B"
                                  -----------


          MEMORANDUM OF LEASE COMMENCEMENT AND ACCEPTANCE OF PREMISES
                         (First Floor Expansion Space)

     This Memorandum of Lease Commencement and Acceptance of Premises is made as
of December 7, 2000, by OKC Real Estate Investments, Inc. ("Landlord"), having
an office c/o TC Oklahoma City, Inc., 1601 N.W. Expressway, Suite 1200, Oklahoma
City, Oklahoma 73118, and AT&T Corp. ("Tenant"), having an office at 150 Mt.
Airy Road, Basking Ridge, New Jersey 07920. Landlord and Tenant agree to and
acknowledge the following matters:

     1.   Landlord and Tenant have entered into a Lease Agreement (the "Lease"),
covering certain lease premises commonly known as Suite 200, containing
approximately 103,500 rentable square feet (the "Existing Space" and "Expansion
Space" as defined in the Lease), in the property known as Quail Commerce Centre
located at 3201 Quail Springs Parkway as more particularly described in the
Lease (the "Premises"). Tenant took possession of and accepted (a) the Existing
Space on June 5, 2000, as set forth in the previous Memorandum of Lease
Commencement and Acceptance of Premises dated June 3, 2000 and (b) that portion
of the Expansion Space located on the second floor on December 10, 2000, as set
forth in the previous Memorandum of Lease Commencement and Acceptance of
Premises dated December 7, 2000.

     2.   Tenant has now also accepted possession of that portion of the
Expansion Space located on the first floor (the "First Floor Space") pursuant to
the Lease as suitable for the purpose for which they were leased. The Lease term
for the First Floor Space commences on December 28, 2000. The termination date
of the Lease term, excluding renewals and extensions, is November 30, 2010.

     3.   The beginning monthly rent presently payable under the Lease for the
First Floor Space is $39,250.00 with adjustments as described in the Lease.
Tenant is not entitled to rent concessions of any kind under the Lease for the
First Floor Space except as follows: None. No rent or other sum payable under
                                     ----
the Lease has been paid more than one month in advance.

     4.   Any improvements required by the terms of the Lease to be made by
Landlord for the First Floor Space have been completed to the satisfaction of
Tenant in all respects, and Landlord has fulfilled all of its duties for the
First Floor Space under the Lease.

     5.   The Lease is valid and in full force and effect, and, to the best of
Tenant's knowledge, neither Landlord nor Tenant is in default thereunder. Tenant
has no defense, setoff or counterclaim against Landlord arising out of the Lease
or in any way relating thereto, or arising out of any other transaction between
Tenant and Landlord, and no event has occurred and no condition exists, which
with the giving of notice or the passage of time, or both, will constitute a
default under the Lease.

     IN WITNESS WHEREOF, Landlord and Tenant has executed this Memorandum as of
December 7, 2000.


TENANT:                                      LANDLORD:


AT&T CORP.                                   OKC REAL ESTATE INVESTMENTS, INC.


By: /s/ Bill Blanton                         By: /s/ Shaun W. Frankfurt
    ---------------------------                  -----------------------------

Printed Name: Bill Blanton                   Printed Name: Shaun W. Frankfurt
              -----------------                            -------------------

Title: Southern Area Director                Title: President
       ------------------------                     --------------------------

                                  Page 1 of 3
<PAGE>

          MEMORANDUM OF LEASE COMMENCEMENT AND ACCEPTANCE OF PREMISES
                        (Second Floor Expansion Space)

     This Memorandum of Lease Commencement and Acceptance of Premises is made as
of Dec. 7, 2000, by OKC Real Estate Investments, Inc. ("Landlord"), having an
office c/o TC Oklahoma City, Inc., 1601 N.W. Expressway, Suite 1200, Oklahoma
City, Oklahoma 73118, and AT&T Corp. ("Tenant"), having an office at 150 Mt.
Airy Road, Basking Ridge, New Jersey 07920. Landlord and Tenant agree to and
acknowledge the following matters:

     1.   Landlord and Tenant have entered into a Lease Agreement (the "Lease"),
covering certain lease premises commonly known as Suite 200, containing
approximately 103,500 rentable square feet (the "Existing Space" and "Expansion
Space" as defined in the Lease), in the property known as Quail Commerce Centre
located at 3201 Quail Springs Parkway as more particularly described in the
Lease (the "Premises"). Tenant took possession of and accepted the Existing
Space on June 5, 2000, as set forth in the previous Memorandum of Lease
Commencement and Acceptance of Premises dated June 3, 2000.

     2.   Tenant has now also accepted possession of that portion of the
Expansion Space located on the second floor (the "Second Floor Space") pursuant
to the Lease as suitable for the purpose for which they were leased. The Lease
term for the Second Floor Space commences on December 10, 2000. The termination
date of the Lease term, excluding renewals and extensions, is November 30, 2010.

     3.   The beginning monthly rent presently payable under the Lease for the
Second Floor Space is $39,250.00 with adjustments as described in the Lease.
Tenant is not entitled to rent concessions of any kind under the Lease for the
Second Floor Space except as follows: None. No rent or other sum payable under
                                      ----
the Lease has been paid more than one month in advance.

     4.   Any improvements required by the terms of the Lease to be made by
Landlord for the Second Floor Space have been completed to the satisfaction of
Tenant in all respects, and Landlord has fulfilled all of its duties for the
Second Floor Space under the Lease.

     5.   The Lease is valid and in full force and effect, and, to the best of
Tenant's knowledge, neither Landlord nor Tenant is in default thereunder. Tenant
has no defense, setoff or counterclaim against Landlord arising out of the Lease
or in any way relating thereto, or arising out of any other transaction between
Tenant and Landlord, and no event has occurred and no condition exists, which
with the giving of notice or the passage of time, or both, will constitute a
default under the Lease.

     IN WITNESS WHEREOF, Landlord and Tenant has executed this Memorandum as of
Dec 7, 2000.


TENANT:                                 LANDLORD:

AT&T CORP.                              OKC REAL ESTATE INVESTMENTS, INC.

By: /s/ Bill Blanton                    By: /s/ Shaun W. Frankfurt
    -------------------------               -------------------------------

Printed Name: Bill Blanton              Printed Name: Shaun W. Frankfurt
              ---------------                         ---------------------

Title: Southern Area Director           Title: President
       ----------------------                  ----------------------------

                                  Page 2 of 3
<PAGE>

          MEMORANDUM OF LEASE COMMENCEMENT AND ACCEPTANCE OF PREMISES

          This Memorandum of Lease Commencement and Acceptance of Premises is
made as of June 3, 2000 by OKC Real Estate Investments, Inc. ("Landlord"),
having an office c/o TC Oklahoma City, Inc., 1601 N. W. Expressway, Suite 1200,
Oklahoma City, Oklahoma 73118, and AT&T Corp. ("Tenant"), having an office at
150 Mt. Airy Road, Basking Ridge, New Jersey 07920. Landlord and Tenant agree to
and acknowledge the following matters:

          1.   Landlord and Tenant have entered into a Lease Agreement (the
"Lease"), covering certain leased premises commonly know as Suite 200,
containing approximately 103,500 rentable square feet (the "Existing Space" and
"Expansion Space" as defined in the Lease), in the property know as Quail
Commerce Centre located at 3201 Quail Springs Parkway as more particularly
described in the Lease (the "Premises").

          2.   Tenant has accepted possession of the Existing Space pursuant to
the Lease as suitable for the purpose for which they were leased. The Lease term
for the Existing Space commences on June 5, 2000. The termination date of the
Lease term, excluding renewals and extensions, is November 30, 2010.

          3.   The beginning monthly rent presently payable under the Lease for
the Existing Space is $25,000.00 with adjustments as described in the Lease.
Tenant is not entitled to rent concessions of any kind under the Lease for the
Existing Space except as follows: None. No rent or other sum payable under the
                                  ----
Lease has been paid more than one month in advance.

          4.   Any improvements required by the terms of the Lease to be made by
Landlord for the Existing Space have been completed to the satisfaction of
Tenant in all respects, and Landlord has fulfilled all of its duties for the
Existing Space under the Lease.

          5.   The Lease is valid and in full force and effect, and, to the best
of Tenant's knowledge, neither Landlord nor Tenant is in default thereunder.
Tenant has no defense, setoff or counterclaim against Landlord arising out of
the Lease or in any way relating thereto, or arising out of any other
transaction between Tenant and Landlord, and no event has occurred and no
condition exists, which with the giving of notice or the passage of time, or
both, will constitute a default under the Lease.

     IN WITNESS WHEREOF, Landlord and Tenant has executed this Memorandum as of
June 3, 2000.

TENANT:                                LANDLORD:

AT&T CORP.                             OKC REAL ESTATE INVESTMENTS, INC.

By: /s/ Charles B. Alexander           By: /s/ Shaun W. Frankfurt
   ---------------------------------      ---------------------------------

Printed Name: Charles B. Alexander     Printed Name: Shaun W. Frankfurt
             -----------------------                -----------------------

Title:        Asset Manager            Title:        President
      ------------------------------         ------------------------------

                                  Page 3 of 3
<PAGE>

                                 EXHIBIT "C"
                                 -----------


                                 December __, 2000



Wells Fund XII-REIT Joint Venture Partnership
6200 The Corners Parkway, Suite 250
Norcross, GA 30092

     RE:  Agreement for the Purchase and Sale of Property dated June 22, 2000,
          as amended by the First Amendment to Real Estate Purchase Contract
          effective as of December 29, 2000 (Quail Commerce Centre, Oklahoma
          City, Oklahoma), together the "Contract"

Ladies and Gentlemen:

     This will confirm that Trammell Crow Company does hereby guarantee the
performance by OKC Real Estate Investments, Inc. ("OKC") of the "Post-Closing
Obligations", as  defined in the First Amendment to Real Estate Purchase
Contract described above.

     This Guaranty is an absolute and unconditional guaranty payment (if
necessary) and of performance.  The liability of Guarantor is co-extensive with
that of OKC and also joint and several and this Guaranty shall be enforceable
against the undersigned without the necessity of any suit or proceeding against
OKC and without the necessity of any notice of non-payment, non-performance or
non-observance or of any notice of acceptance of this Guaranty or of any other
notice or demand to which the undersigned might otherwise be entitled, all of
which we hereby expressly waive. The validity of this Guaranty and the
obligations hereunder shall in no way be terminated, affected, diminished or
impaired by reason of (a) the assertion or the failure to assert by you against
OKC of any of the rights or remedies otherwise available to you, or (b) any non-
liability of OKC under the Contract, whether by insolvency, discharge in
bankruptcy, or any other defect or defense which may now or hereafter exist in
favor of OKC.


               [add appropriate signature block for TCC officer]