Document:

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                                                                   EXHIBIT 10.37

                       MEMBERSHIP INTEREST SALE AGREEMENT
                                 (MADISON PLACE)

                                 BY AND BETWEEN

                          CLARK/CARR INVESTMENTS, LLC,
                      A MARYLAND LIMITED LIABILITY COMPANY,

                                    AS SELLER

                                       AND

                              COLUMBIA EQUITY, LP,
                         A VIRGINIA LIMITED PARTNERSHIP,

                                  AS PURCHASER

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                                TABLE OF CONTENTS
                                -----------------

<TABLE>
<S>                 <C>                                                                                         <C>
ARTICLE I  THE SALE...............................................................................................1

         1.1        Sale of Membership Interest...................................................................1
         1.2        Purchase Price................................................................................1

ARTICLE II  REPRESENTATIONS AND COVENANTS.........................................................................2

         2.1        Representations by Purchaser..................................................................2
         2.2        Representations by Seller.....................................................................3
         2.3        Covenants of Purchaser........................................................................4
         2.4        Covenants of Seller...........................................................................4

ARTICLE III  CONDITIONS PRECEDENT TO THE CLOSING..................................................................5

         3.1        Conditions to Purchaser's Obligations.........................................................5
         3.2        Conditions to Seller's Obligations............................................................6

ARTICLE IV  CLOSING AND CLOSING DOCUMENTS.........................................................................6

         4.1        Closing.......................................................................................6
         4.2        Seller's Deliveries...........................................................................6
         4.3        Purchaser's Deliveries........................................................................7
         4.4        Fees and Expenses; Closing Costs..............................................................7
         4.5        Adjustments...................................................................................7

ARTICLE V  MISCELLANEOUS..........................................................................................8

         5.1        Notices.......................................................................................8
         5.2        Entire Agreement; Modifications and Waivers; Cumulative Remedies..............................8
         5.3        Exhibits......................................................................................9
         5.4        Successors and Assigns........................................................................9
         5.5        Article Headings..............................................................................9
         5.6        Governing Law.................................................................................9
         5.7        Counterparts..................................................................................9
         5.8        Survival......................................................................................9
         5.9        Severability..................................................................................9
         5.10       Attorneys' Fees...............................................................................9

EXHIBITS

         A          Assignment and Assumption Agreement
</TABLE>

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                       MEMBERSHIP INTEREST SALE AGREEMENT

        THIS MEMBERSHIP INTEREST SALE AGREEMENT (this "Agreement") is made as of
this 31st day of March, 2005 by and between Clark/Carr Investments, LLC, a
Maryland limited liability company ("Seller"); and Columbia Equity, LP, a
Virginia limited partnership ("Purchaser").

                                    RECITALS

        A.      Madison Place, LLC, a Virginia limited liability company (the
"LLC") is the owner of certain land located at 500 Montgomery Street in
Alexandria, Virginia (the "Land") and the office building and related
improvements located thereon (the "Improvements"), which Land and Improvements
(collectively, the "Property") are more commonly known as Madison Place.

        B.      Carr Capital Madison, LLC, a Virginia limited liability company
("Carr Madison") is the record and beneficial owner of Fifteen percent (15%) of
the membership interests in the LLC.

        C.      Seller is the record and beneficial owner of Twenty and 00/100
percent (20%) of the membership interests in Carr Madison (the "Membership
Interest").

        D.      Seller desires to sell the Membership Interest to Purchaser, on
the terms and conditions hereinafter set forth.

        E.      Purchaser desires to purchase the Membership Interest from
Seller, on the terms and conditions hereinafter set forth.

                                   AGREEMENT

        NOW, THEREFORE, for and in consideration of the mutual covenants herein
contained, the parties hereto agree as follows:

                                   ARTICLE I
                                    THE SALE

        1.1     Sale of Membership Interest. Seller agrees to sell, transfer,
assign and convey the Membership Interest to Purchaser, and Purchaser agrees to
purchase and accept transfer of the Membership Interest pursuant to the terms
and conditions set forth in this Agreement. The Membership Interest shall be
transferred to Purchaser free and clear of all liens, encumbrances, security
interests, prior assignments or conveyances, conditions, restrictions, voting
agreements, claims, and any other matters affecting title thereto (other than
the Carr Madison Operating Agreement (the "Carr Madison Operating Agreement")).

        1.2     Purchase Price. The purchase price (the "Purchase Price") for
which Seller agrees to sell and assign the Membership Interest to Purchaser, and
which Purchaser agrees to pay to Seller, subject to the terms of this Agreement,
shall be Four Hundred Twenty-Six Thousand Five Hundred Seventy-Two Dollars
($426,572).

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                                   ARTICLE II
                          REPRESENTATIONS AND COVENANTS

        2.1     Representations by Purchaser. Purchaser hereby represents and
warrants to Seller that the following statements are true, correct, and complete
in every material respect as of the date of this Agreement and will be true,
correct, and complete as of the Closing Date:

                (a)     Organization and Power. Purchaser is duly organized and
validly existing as a limited partnership under the laws of the Commonwealth of
Virginia, and has full right, power, and authority to enter into this Agreement
and to perform all of its obligations under this Agreement; and, the execution
and delivery of this Agreement and the performance by Purchaser of its
obligations under this Agreement have been duly authorized by all requisite
action of Purchaser and require no further action or approval of Purchaser's
partners or of any other individuals or entities in order to constitute this
Agreement as a binding and enforceable obligation of Purchaser.

                (b)     Noncontravention. Neither the entry into nor the
performance of, or compliance with, this Agreement by Purchaser has resulted, or
will result, in any violation of, or default under, or result in the
acceleration of, any obligation under the partnership agreement of Purchaser, or
any mortgage, indenture, lien agreement, note, contract, permit, judgment,
decree, order, restrictive covenant, statute, rule, or regulation applicable to
Purchaser.

                (c)     Litigation. There is no action, suit, or proceeding,
pending or known to be threatened, against or affecting Purchaser in any court
or before any arbitrator or before any federal, state, municipal, or other
governmental department, commission, board, bureau, agency or instrumentality
which (i) in any manner raises any question affecting the validity or
enforceability of this Agreement, (ii) would reasonably be expected to
materially and adversely affect the business, financial position, or results of
operations of Purchaser, (iii) would reasonably be expected to materially and
adversely affect the ability of Purchaser to perform its obligations hereunder,
or under any document to be delivered pursuant hereto.

                (d)     Consents. Each consent, approval, authorization, order,
license, certificate, permit, registration, designation, or filing by or with
any governmental agency or body necessary for the execution, delivery, and
performance of this Agreement or the transactions contemplated hereby by
Purchaser has been obtained.

                (e)     Bankruptcy with respect to Purchaser. No Act of
Bankruptcy has occurred with respect to Purchaser. As used herein, "Act of
Bankruptcy" shall mean if a party hereto shall (A) apply for or consent to the
appointment of, or the taking of possession by, a receiver, custodian, trustee
or liquidator of itself or of all or a substantial part of its property, (B)
admit in writing its inability to pay its debts as they become due, (C) make a
general assignment for the benefit of its creditors, (D) file a voluntary
petition or commence a voluntary case or proceeding under the Federal Bankruptcy
Code (as now or hereafter in effect), (E) be adjudicated bankrupt or insolvent,
(F) file a petition seeking to take advantage of any other law relating to
bankruptcy, insolvency, reorganization, winding-up or composition or adjustment
of debts, (G) fail to controvert in a timely and appropriate manner, or
acquiesce in writing to, any petition filed

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against it in an involuntary case or proceeding under the Federal Bankruptcy
Code (as now or hereafter in effect), or (H) take any action for the purpose of
effecting any of the foregoing.

                (f)     Brokerage Commission. Purchaser has not engaged the
services of, nor has it or will it or Seller become liable to, any real estate
agent, broker, finder or any other person or entity for any brokerage or
finder's fee, commission or other amount with respect to the transactions
described herein on account of any action by Purchaser. Purchaser hereby agrees
to indemnify and hold Seller and its employees, directors, members, partners,
affiliates and agents harmless against any actual claims, liabilities, damages
or expenses arising out of a breach of the foregoing. This indemnification shall
survive Closing or any termination of this Agreement.

        2.2     Representations by Seller. Seller hereby represents and warrants
unto Purchaser that each and every one of the following statements is true,
correct, and complete in every material respect as of the date of this Agreement
and will be true, correct, and complete as of the Closing Date:

                (a)     Organization and Power. Seller is duly organized,
validly existing, and in good standing as a limited liability company under the
laws of the State of Maryland. Seller has full right, power, and authority to
enter into this Agreement and to perform all of its obligations under this
Agreement; and the execution and delivery of this Agreement and the performance
by Seller of its obligations hereunder have been duly authorized by all
requisite action of Seller and require no further action or approval of Seller's
members or managers or of any other individuals or entities in order to
constitute this Agreement as a binding and enforceable obligation of Seller.

                (b)     Noncontravention. To Seller's actual knowledge, neither
the entry into nor the performance of, or compliance with, this Agreement by
Seller has resulted, or will result, in any material violation of, or material
default under, or result in the acceleration of, any material obligation under
any material limited liability company agreement, operating agreement,
regulation, mortgage, indenture, lien agreement, note, contract, permit,
judgment, decree, order, restrictive covenant, statute, rule, or regulation
applicable to Seller or, to the actual knowledge of Seller, to the Membership
Interest.

                (c)     Litigation. There is no action, suit, claim, or
proceeding pending or, to the actual knowledge of Seller, threatened against
Seller, or, to the actual knowledge of Seller, the Membership Interest in any
court, or before any arbitrator, or before any federal, state, municipal or
other governmental department, commission, board, bureau, agency or
instrumentality which (A) would reasonably be expected to materially and
adversely affect the ability of Seller to perform its obligations hereunder, or
under any document to be delivered pursuant hereto, (B) would reasonably be
expected to create a lien on the Membership Interest, any part thereof, or any
interest therein, or (C) would reasonably be expected to materially and
adversely affect the Membership Interest, any part thereof, or any interest
therein.

                (d)     Good Title. (A) Seller has good title to the Membership
Interest on the date hereof and will have good title to the Membership Interest
on the Closing Date (other than the Carr Madison Operating Agreement), (B) the
Membership Interest on the date hereof is and on the Closing Date will be free
and clear of all liens, encumbrances, pledges, voting agreements and security
interests whatsoever (other than the Carr Madison Operating Agreement), and (C)

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Seller has not granted any other person or entity an option to purchase or a
right of first refusal upon the Membership Interest nor are there any agreements
or understandings between Seller and any other person or entity with respect to
the disposition of the Membership Interest (other than the Carr Madison
Operating Agreement).

                (e)     Bankruptcy with respect to Seller. No Act of Bankruptcy
has occurred with respect to Seller.

                (f)     Brokerage Commission. Seller has not engaged the
services of, nor has it or will it or Purchaser become liable to, any real
estate agent, broker, finder or any other person or entity for any brokerage or
finder's fee, commission or other amount with respect to the transactions
described herein on account of any action by Seller. Seller hereby agrees to
indemnify and hold Purchaser and its employees, directors, members, partners,
affiliates and agents harmless against any actual claims, liabilities, damages
or expenses arising out of a breach of the foregoing. This indemnification shall
survive Closing or any termination of this Agreement.

                (g)     Default. To Seller's actual knowledge, Seller is not in
default under the Carr Madison Operating Agreement.

        2.3     Covenants of Purchaser. Purchaser agrees as follows:

                (a)     Further Acts. In addition to the acts, instruments and
agreements recited herein and contemplated to be performed, executed and
delivered by Purchaser and Seller, Purchaser shall perform, execute, and deliver
or cause to be performed, executed, and delivered at the Closing or after the
Closing, any and all further acts, instruments, and agreements and provide such
further assurances as Seller may reasonably require to consummate the
transactions contemplated hereunder.

        2.4     Covenants of Seller. Seller agrees as follows:

                (a)     Actions Regarding Membership Interest. Except as
otherwise permitted hereby, from the date hereof until the Closing Date, Seller
shall use reasonable commercial efforts not to take any action or fail to take
any action within Seller's control the result of which would (1) have a material
adverse effect on the Membership Interest, Seller's ability to sell, transfer,
assign and convey the Membership Interest to Purchaser or Purchaser's ability to
continue the ownership thereof after the Closing Date or (2) cause any of the
representations and warranties contained in Section 2.2 to be untrue in any
material respects as of the Closing Date.

                (b)     Confidentiality. Seller acknowledges that the matters
relating to the REIT, the initial public offering of REIT securities (the
"IPO"), this Agreement, and the other documents, terms, conditions and
information related thereto (collectively, the "Information") are confidential
in nature. Therefore, Seller covenants and agrees to keep the Information
confidential and will not (except as required by applicable law, regulation or
legal process, and only after compliance with the provisions of this Section
2.4) prior to the IPO, without Purchaser's prior written consent, disclose any
Information in any manner whatsoever; provided, however, that the Information
may be revealed only to Seller's employees, legal counsel and financial
advisors, each of whom shall be informed of the confidential nature of the
Information.

                                     - 4 -
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In the event that Seller or its key employees, legal counsel or financial
advisors (collectively, the "Information Group") are requested pursuant to, or
required by, applicable law, regulation or legal process to disclose any of the
Information, the applicable member of the Information Group will notify
Purchaser promptly so that it may seek a protective order or other appropriate
remedy or, in its sole discretion, waive compliance with the terms of this
Section 2.4. Seller acknowledges that Purchaser's sole remedy for breach of this
Section shall be to seek an order granting injunctive relief in favor of the
REIT and/or Purchaser without proof of actual damages.

                (c)     Further Acts. In addition to the acts, instruments and
agreements recited herein and contemplated to be performed, executed and
delivered by Purchaser and Seller, Seller shall perform, execute, and deliver or
cause to be performed, executed, and delivered, at no additional cost to Seller,
at the Closing or after the Closing, any and all further acts, instruments, and
agreements and provide such further assurances as Purchaser may reasonably
require to consummate the transactions contemplated hereunder.

                                  ARTICLE III
                       CONDITIONS PRECEDENT TO THE CLOSING

        3.1     Conditions to Purchaser's Obligations. In addition to any other
conditions set forth in this Agreement, Purchaser's obligation to consummate the
Closing is subject to the timely satisfaction of each and every one of the
conditions and requirements set forth in this Section 3.1, all of which shall be
conditions precedent to Purchaser's obligations under this Agreement.

                (a)     Seller's Obligations. Seller shall have performed all
obligations of Seller hereunder which are to be performed prior to Closing, and
shall have delivered or caused to be delivered to Purchaser, all of the
documents and other information required of Seller pursuant to Section 4.2.

                (b)     Seller's Representations and Warranties. Seller's
representations and warranties set forth in Section 2.2 shall be true and
correct in all material respects as if made again on the Closing Date, and
Seller shall have executed and delivered to Purchaser at Closing a certificate
to the foregoing effect.

                (c)     No Injunction. On the Closing Date, there shall be no
effective injunction, writ, preliminary restraining order or other order issued
by a court of competent jurisdiction restraining or prohibiting the consummation
of the transactions contemplated hereby.

                (d)     Completion of IPO. The IPO shall have been completed.

                (e)     Other Members Transactions or Consents. Aetna Life
Insurance Company, a Connecticut corporation, and all members of Carr Madison
who have not assigned the entirety of their membership interests in Carr Madison
to Acquirer, shall have consented to the transactions contemplated by this
Agreement.

                (f)     Mortgage Holder Consent. The holder of the mortgage loan
secured by the Property (the "Mortgage Holder") shall have consented to the
transactions contemplated by this Agreement.

                                     - 5 -
<PAGE>

                (g)     Closing. The Closing shall have occurred on or prior to
June 30, 2005.

        3.2     Conditions to Seller's Obligations. In addition to any other
conditions set forth in this Agreement, Seller's obligations to consummate the
Closing is subject to the timely satisfaction of each and every one of the
conditions and requirements set forth in this Section 3.2, all of which shall be
conditions precedent to Seller's obligations under this Agreement.

                (a)     Purchaser's Obligations. Purchaser shall have performed
all obligations of Purchaser hereunder which are to be performed prior to
Closing, and shall have delivered or caused to be delivered to Seller, all of
the documents and other information required of Purchaser pursuant to Section
4.3.

                (b)     Purchaser's Representations and Warranties. Purchaser's
representations and warranties set forth in Section 2.1 shall be true and
correct in all material respects as if made again on the Closing Date, and
Purchaser shall have executed and delivered to Seller at Closing a certificate
to the foregoing effect.

                (c)     No Injunction. On the Closing Date, there shall be no
effective injunction, writ, preliminary restraining order or other order issued
by a court of competent jurisdiction restraining or prohibiting the consummation
of the transactions contemplated hereby.

                (d)     Completion of IPO. The IPO shall have been completed.

                (e)     Other Members Transactions or Consents. Aetna Life
Insurance Company, a Connecticut corporation, and all members of Carr Madison
who have not assigned the entirety of their membership interests in Carr Madison
to Acquirer, shall have consented to the transactions contemplated by this
Agreement.

                (f)     Mortgage Holder Consent. The holder of the mortgage loan
secured by the Property (the "Mortgage Holder") shall have consented to the
transactions contemplated by this Agreement.

                (g)     Closing. The Closing shall have occurred on or prior to
June 30, 2005.

                                   ARTICLE IV
                          CLOSING AND CLOSING DOCUMENTS

        4.1     Closing. The consummation and closing (the "Closing") of the
transactions contemplated under this Agreement shall take place at the offices
of Hunton & Williams LLP, Washington, D.C., or such other place as is mutually
agreeable to the parties, on the date of the closing of the IPO (the "Closing
Date"), or as otherwise set by agreement of the parties; provided, however, that
this Agreement shall terminate if Closing does not occur prior to June 30, 2005.

        4.2     Seller's Deliveries. At the Closing, Seller shall deliver the
following to Purchaser in addition to all other items required to be delivered
to Purchaser by Seller:

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<PAGE>

                (a)     Assignment of Membership Interest. Seller shall have
executed and delivered to Purchaser an Assignment and Assumption Agreement, in
substantially the form of Exhibit A attached hereto (the "Assignment and
Assumption Agreement").

                (b)     Authority Documents. Evidence satisfactory to Purchaser
that the person or persons executing the closing documents on behalf of Seller
has full right, power, and authority to do so.

                (c)     FIRPTA Certificate. An affidavit from Seller certifying
pursuant to Section 1445 of the Internal Revenue Code that Seller is not a
foreign corporation, foreign partnership, foreign trust, foreign estate or
foreign person (as those terms are defined in the Internal Revenue Code and the
Income Tax Regulations promulgated thereunder), in form and substance
satisfactory to Purchaser.

                (d)     Certificate of Representations and Warranties. The
certificate required by Section 3.1(b).

                (e)     Other Documents. Any other document or instrument
reasonably requested by Purchaser or required hereby.

        4.3     Purchaser's Deliveries. At the Closing, Purchaser shall deliver
the following to Seller:

                (a)     Purchase Price. The Purchaser shall deliver funds in the
amount of the Purchase Price to Seller.

                (b)     Assumption of Membership Interest. Purchaser shall have
executed and delivered to Seller an Assignment and Assumption Agreement.

                (c)     Authority Documents. Evidence satisfactory to Seller
that the person or persons executing the closing documents on behalf of
Purchaser have full right, power, and authority to do so.

                (d)     Certificate of Representations and Warranties. The
certificate required by Section 3.2(b).

                (e)     Other Documents. Any other document or instrument
reasonably requested by Seller or required hereby.

        4.4     Fees and Expenses; Closing Costs. Purchaser shall pay all fees,
expenses and closing costs relating to the transactions contemplated by this
Agreement; provided however, that Seller shall pay its own attorneys' and
consultants' fees and expenses.

        4.5     Adjustments. Purchaser shall pay funds in the amount of the
Purchase Price to Seller without any setoffs or adjustments. Carr Madison shall
be managed in accordance with the Carr Madison Operating Agreement in the
ordinary course of business, up to and including the Closing Date. Seller and
Purchaser acknowledge and agree that the LLC and Carr Madison shall make normal
and customary distributions to their respective members up to and through the

                                     - 7 -
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Closing Date, in accordance with the terms of the LLC's Operating Agreement and
the Carr Madison Operating Agreement, respectively. At Closing, the Purchase
Price shall be increased by Three Percent (3%) of the amount, if any, by which
all cash held by or for the benefit of the LLC (excluding lender held reserves,
reserves for leasing and reserves for capital expenditures) exceeds $205,000.

                                   ARTICLE V
                                  MISCELLANEOUS

        5.1     Notices. Any notice provided for by this Agreement and any other
notice, demand, or communication required hereunder shall be in writing and
either delivered in person (including by confirmed facsimile transmission) or
sent by registered or certified mail or overnight courier, return receipt
requested, in a sealed envelope, postage prepaid, and addressed to the party for
which such notice, demand or communication is intended at such party's address
as set forth in this Section. All notices to Purchaser shall be addressed as
follows:

        PURCHASER:

                Columbia Equity, LP
                c/o Carr Capital Corporation
                1750 H Street, NW, Suite 500
                Washington, DC 20005
                Telephone: (202) 303-3060
                Facsimile: (202) 303-3078
                Attn: Oliver T. Carr, III

Seller's address for all purposes under this Agreement shall be the following:

        SELLER:

                Clark/Carr Investments, LLC
                c/o CEI Realty, Inc.
                7500 Old Georgetown Road, 15th Floor
                Bethesda, MD 20814
                Attn: Rebecca L. Owen, Esq.

Any address or name specified above may be changed by a notice given by the
addressee to the other party. Any notice, demand or other communication shall be
deemed given and effective as of the date of delivery in person or receipt set
forth on the return receipt. The inability to deliver because of changed address
of which no notice was given, or rejection or other refusal to accept any
notice, demand or other communication, shall be deemed to be receipt of the
notice, demand or other communication as of the date of such attempt to deliver
or rejection or refusal to accept.

        5.2     Entire Agreement; Modifications and Waivers; Cumulative
Remedies. This Agreement supersedes any existing letter of intent between the
parties, constitutes the entire agreement among the parties hereto and may not
be modified or amended except by instrument

                                     - 8 -
<PAGE>

in writing signed by the parties hereto, and no provisions or conditions may be
waived other than by a writing signed by the party waiving such provisions or
conditions. No delay or omission in the exercise of any right or remedy accruing
to Seller or Purchaser upon any breach under this Agreement shall impair such
right or remedy or be construed as a waiver of any such breach theretofore or
thereafter occurring. The waiver by Seller or Purchaser of any breach of any
term, covenant, or condition herein stated shall not be deemed to be a waiver of
any other breach, or of a subsequent breach of the same or any other term,
covenant, or condition herein contained. All rights, powers, options, or
remedies afforded to Seller or Purchaser either hereunder or by law shall be
cumulative and not alternative, and the exercise of one right, power, option, or
remedy shall not bar other rights, powers, options, or remedies allowed herein
or by law, unless expressly provided to the contrary herein.

        5.3     Exhibits. All exhibits referred to in this Agreement and
attached hereto are hereby incorporated in this Agreement by reference.

        5.4     Successors and Assigns. Except as set forth in this Article,
this Agreement may not be assigned by Purchaser or Seller without the prior
approval of the other party hereto. This Agreement shall be binding upon, and
inure to the benefit of, Seller, Purchaser, and their respective legal
representatives, successors, and permitted assigns.

        5.5     Article Headings. Article headings and article and section
numbers are inserted herein only as a matter of convenience and in no way
define, limit, or prescribe the scope or intent of this Agreement or any part
hereof and shall not be considered in interpreting or construing this Agreement.

        5.6     Governing Law. This Agreement shall be construed and interpreted
in accordance with the laws of the Commonwealth of Virginia, without regard to
conflicts of laws principles.

        5.7     Counterparts. This Agreement may be executed in any number of
counterparts and by any party hereto on a separate counterpart, each of which
when so executed and delivered shall be deemed an original and all of which
taken together shall constitute but one and the same instrument.

        5.8     Survival. All representations and warranties contained in this
Agreement, and all covenants and agreements contained in the Agreement which
contemplate performance after the Closing Date shall survive the Closing.

        5.9     Severability. In case any one or more of the provisions
contained in this Agreement shall for any reason be held to be invalid, illegal,
or unenforceable in any respect, such invalidity, illegality, or
unenforceability shall not affect any other provision hereof, and this Agreement
shall be construed as if such invalid, illegal, or unenforceable provision had
never been contained herein.

        5.10    Attorneys' Fees. Should a party employ an attorney or attorneys
to enforce any of the provisions hereof or to protect its interest in any manner
arising under this Agreement, or to recover damages for breach of this
Agreement, any non-prevailing party in any action pursued in a court of
competent jurisdiction (the finality of which is not legally contested) shall
pay to the

                                     - 9 -
<PAGE>

prevailing party all reasonable costs, damages, and expenses, including
reasonable attorneys' fees, expended or incurred in connection therewith.

                            [SIGNATURE PAGE FOLLOWS]

                                     - 10 -
<PAGE>

        IN WITNESS WHEREOF, this Agreement has been entered into effective as of
the date first above written.

<TABLE>
<CAPTION>
                                                SELLER:
                                                -------
<S>                                             <C>
                                                Clark/Carr Investments, LLC, a Maryland limited
                                                liability company

                                                By: CEI Realty, Inc., a District of Columbia
                                                    corporation, its manager

                                                    By: /s/ Lawrence C. Nussdorf
                                                       --------------------------------------------
                                                    Name: Lawrence C. Nussdorf
                                                    Title: President

                                                PURCHASER:
                                                ----------

                                                Columbia Equity, LP, a Virginia limited partnership

                                                By: Columbia Equity Trust, Inc., a Maryland
                                                    corporation, its general partner

                                                    By: /s/ Oliver T. Carr, III
                                                       --------------------------------------------
                                                    Name: Oliver T. Carr, III
                                                    Title: Chairman and Chief Executive Officer
</TABLE>

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                                    EXHIBIT A

                       ASSIGNMENT AND ASSUMPTION AGREEMENT

Clark/Carr Investments LLC, a Maryland Limited liability company ("Assignor"),
for good and valuable consideration paid to the Assignor by Columbia Equity, LP,
a Virginia limited partnership ("Assignee"), pursuant to the Membership Interest
Sale Agreement dated as of ____________, 2005, by and between
_______________________ (the "Agreement") and for other good and valuable
consideration, the receipt and adequacy of which are hereby acknowledged, does
hereby sell, assign, transfer, convey and deliver to the Assignee, its
successors and assigns, good and indefeasible title to the Membership Interest,
free and clear of all liens, encumbrances, security interests, prior
assignments, voting agreements, conditions, restrictions, pledges, claims, and
other matters affecting title thereto, subject to the Carr Madison Operating
Agreement. Assignee does hereby accept the foregoing Assignment and assumes and
agrees to be responsible for all liabilities and obligations under the Carr
Madison Operating Agreement from and after the date hereof relating to the
Membership Interest.

Assignee agrees to indemnify, hold harmless and defend Assignor from and against
all claims, damages, debts, liabilities and obligations arising from business
done, transactions entered into or other events occurring after the date hereof
with respect to the Membership Interest or the Carr Madison Operating Agreement.

Capitalized terms used but not defined herein shall have the respective meanings
ascribed to them in the Agreement.

IN WITNESS WHEREOF, the parties hereto have caused this Assignment and
Assumption Agreement to be signed by a duly authorized officer of each, this __
day of _____, 2005.

                                                ASSIGNOR:

                                                _______________________, a

                                                -----------------------

                                                By:
                                                    ----------------------------
                                                Name:
                                                      --------------------------
                                                Title:
                                                       -------------------------

                                                ASSIGNEE:

                                                _________________________, a

                                                -----------------------

                                                By:
                                                    ----------------------------
                                                Name:
                                                      --------------------------
                                                Title:
                                                       -------------------------<PAGE>

                                                                   Exhibit 10.39
                      [Carr Capital Corporation Letterhead]

                                                       May 23, 2005

VIA OVERNIGHT MAIL

Aetna Life Insurance Company
151 Farmington Avenue, RT 11
Hartford, Connecticut 06156-9642
Telephone: (860) 273-1781
Attn: Peter S. Atwood

Copy To:

Bingham McCutchen LLP
One State Street
Hartford, Connecticut 06103
Telephone: (860) 240-2759
Attn: R. Jeffrey Smith, Esq.

        RE:     EXERCISE OF RIGHT TO ACQUIRE A PORTION OF THE OWNERSHIP INTEREST
                OF AETNA LIFE INSURANCE COMPANY, A CONNECTICUT CORPORATION
                ("AETNA"), IN ATRIUM BUILDING, LLC, A VIRGINIA LIMITED LIABILITY
                COMPANY ("ATRIUM")

Ladies and Gentlemen:

        Reference is made to that certain Atrium Building, LLC Operating
Agreement, dated as of May 25, 2004 (the "OPERATING AGREEMENT"), by and among
Carr Capital Atrium, LLC, a Virginia limited liability company ("CCA"), and
Aetna, as Members, and Carr Capital Real Estate Investments, LLC, a Virginia
limited liability company ("CCREI"), as Manager. All capitalized terms used
herein, but not defined herein, shall have the respective meanings given to them
in the Operating Agreement.

        Columbia Equity, LP, a Virginia limited partnership (the "LP"), in its
capacity as an Affiliate of a Carr REIT, is hereby providing you with notice of
its intention to exercise its right to purchase forty percent (40%) of Aetna's
Interest in Atrium (the "PURCHASED INTEREST"), pursuant to the Section 6.8 of
the Operating Agreement and that letter agreement dated as of May 17, 2005, by
and between Aetna, CCA and CCREI (the "PURCHASE TRANSACTION"). The Purchase
Transaction will occur on the date of, and is contingent upon, the initial
public offering (the "IPO") of the common shares of Columbia Equity Trust, Inc.,
a Maryland corporation (the "CARR REIT"), which IPO is currently expected to
occur on or about June 25, 2005.

<PAGE>

        The price for the Purchase Interest shall be the sum of (a) forty
percent (40%) of Aetna's initial Capital Contribution, plus (b) the amount that
will provide Aetna with an IRR of fourteen percent (14%) on the amount paid to
Aetna pursuant to clause (a).

        Please do not hesitate to call Ed Schulze of Carr Capital Corporation at
(202) 303-3072 with any questions related to the Purchase Transaction.

                                                Very truly yours,

                                                COLUMBIA EQUITY, LP, a Virginia
                                                limited partnership

                                                   By:  Columbia Equity Trust,
                                                   Inc., a Maryland corporation,
                                                   its general partner

                                                   By: /s/  Oliver T. Carr, III
                                                     ---------------------------
                                                   Name: Oliver T. Carr, III
                                                   Title: Chairman and Chief
                                                          Executive Officer

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