Document:

<PAGE>

                                                                   Exhibit 10.12

                               FIRST AMENDMENT TO
                     REAL ESTATE PURCHASE AND SALE AGREEMENT

     This First Amendment to Real Estate Purchase and Sale Agreement (the "First
Amendment") is made and entered into as of November 21, 2005 by and among 1025
Vermont Investors, L.L.C., a Delaware limited liability company (the "Seller"),
and Columbia Equity Trust, Inc., a Maryland corporation (the "Buyer") with
reference to the following facts:

                                    RECITALS:

     A. Buyer and Seller have entered into that certain Real Estate Purchase and
Sale Agreement dated as of November 11, 2005 (the "Agreement")(all capitalized
terms not defined in this First Amendment shall have the meanings ascribed to
them in the Agreement); and

     B. Both parties desire to further amend and modify the Agreement as set
forth below.

     NOW, THEREFORE, for and in consideration of the agreements and obligations
hereinafter set forth and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged by the parties hereto, Buyer and
Seller agree as follows:

                                      TERMS

1. Purchase Price. The Purchase Price (as defined in the Contract) is changed to
THIRTY-FOUR MILLION FIFTY THOUSAND and 00/100 DOLLARS ($34,050,000.00).

2. Due Diligence Period. Buyer shall have no further right to terminate the
Agreement pursuant to Section 5(b) thereof.

3. Rent Escrow. At Closing, Seller shall place TWO HUNDRED FIFTY-THREE THOUSAND
EIGHT HUNDRED FORTY-EIGHT and 29/100 DOLLARS ($253,848.29) in escrow (the "Rent
Escrow") with Commercial Title Group, Inc. to compensate Buyer for certain rent
abatements and rent escalation adjustments not previously disclosed to Buyer.
Buyer shall be entitled to withdraw funds held in the Rent Escrow after Closing
from time-to-time, at Buyer's sole discretion, as equivalent rent payments

4. Tenant Improvement and Leasing Commissions Escrow. In the event Seller has
not fulfilled its obligation pursuant to Section 9(a) (v) of the Agreement to
pay any leasing commissions, payments for tenant improvements or tenant
allowances in connection with any Existing Leases that Seller is required to pay
on or before Closing, Seller shall place a reasonable estimate of such unpaid
amounts, as agreed by Seller and Buyer, in escrow (the "TI and Leasing
Commissions Escrow") with Commercial Title Group, Inc. at Closing. Seller shall
be entitled to withdraw funds held in the TI and Leasing Commissions Escrow from
time-to-time after Closing to fund such unpaid leasing commissions, payments for
tenant improvements or tenant allowances in connection with Existing Leases,
only with Buyer's written consent, not to be unreasonably withheld.
Notwithstanding the foregoing and regardless of the amount placed in the TI and
Leasing Commissions Escrow, Seller shall be solely responsible to pay any
leasing commissions, payments for tenant improvements or tenant allowances in
connection with any Existing Leases except for leasing commissions, tenant
improvements or tenant allowances with

<PAGE>

respect to renewals or extensions of Modified Leases or New Leases entered into
after the Effective Date and consented to by Buyer.

5. Additions to Amounts in Rent Escrow. In the event that delays in completing
any tenant improvements for Existing Leases by Seller result in delays in the
occupancy dates, as shown on the Rent Roll attached as Exhibit E to the
Agreement, and reduce the rental and reimbursement income to the Buyer by TEN
THOUSAND DOLLARS ($10,000) or more, Seller shall pay the reasonably estimated
amount of such reduction, as agreed by Seller and Buyer, into the Rent Escrow at
Closing.

6. Environmental Remediation. Seller shall remediate the mold discovered in the
Property's electrical room by Buyer's environmental consultant, according to the
guidelines prepared by Buyer's environmental consultant, to Buyer's reasonable
satisfaction prior to Closing at Seller's sole cost and expense.

7. Ratification. Except as modified by this First Amendment, the Agreement
remains in full force and effect.

8. Counterparts. This First Amendment may be executed in multiple counterparts,
each of which shall be deemed an original, and all of which, taken together,
shall constitute one and the same instrument.

                                        2

<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this First Amendment
as of the date first set forth above.

                                        SELLER:

                                        1025 VERMONT INVESTORS, L.L.C., a
                                        Delaware limited liability company

                                        By: Vermont Avenue Managing Associates,
                                            L.L.C.,
                                            a Delaware limited liability
                                            company,
                                            Managing Member

                                        By: /s/ Andrew J. Czekaj
                                            ------------------------------------
                                            Andrew J Czekaj, Manager

                                        BUYER:

                                        COLUMBIA EQUITY TRUST, INC., a Maryland
                                        corporation

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

                                        3<PAGE>

                                                                   Exhibit 10.13

Record and return to:

Principal Life Insurance Company
c/o Principal Real Estate Investors, LLC, Servicer
801 Grand Avenue
Des Moines, Iowa 50392-1450
ATTN: Beth Jensen

                       CONSENT TO TRANSFER AND ASSUMPTION
                              AND LOAN MODIFICATION

                                   LOAN 754150

     THIS CONSENT TO TRANSFER AND ASSUMPTION AND LOAN MODIFICATION ("Consent")
is entered into effective as of January 12, 2006 among (i) PRINCIPAL LIFE
INSURANCE COMPANY, an Iowa corporation ("Lender"); (ii) ROBERT L. GORHAM, an
attorney licensed to practice law in the District of Columbia and a member in
good standing in the District of Columbia Bar with a law office at Holland &
Knight LLP, 2099 Pennsylvania Avenue, N.W., Suite 100, Washington, D.C. 20006
and JOSEPH P. WHITEBREAD, JR., an attorney licensed to practice law in the
District of Columbia and a member in good standing in the District of Columbia
Bar with a law office at Holland & Knight LLP, 2099 Pennsylvania Avenue, N.W.,
Suite 100, Washington, D.C. 20006, collectively, as ("Trustee"); (iii) 1025
VERMONT INVESTORS, L.L.C., a Delaware limited liability company ("Current
Borrower"); and (iv) 1025 VERMONT AVENUE, LLC, a Virginia limited liability
company ("Proposed Borrower").

                                   WITNESSETH:

     WHEREAS, Lender provided Current Borrower a loan (the "Loan") in the
original principal amount of $19,000,000.00 evidenced by a Secured Promissory
Note dated December 20, 2004, from Current Borrower to Lender (the "Note")
secured by a Deed of Trust, Security Agreement and Assignment of Rents dated
December 20, 2004 (the "Mortgage") encumbering the property more particularly
described in Exhibit A, attached hereto and incorporated herein (the "Property")
and recorded in Document 2004172598, among the land records of the Washington,
District of Columbia Recorder of Deeds, an Assignment of Leases and Rents dated
December 20, 2004 (the "Assignment of Leases and Rents") and recorded in
Document 2004172599, among the land records of the Washington, District of
Columbia Recorder of Deeds, and a Property Reserves Agreement dated December 20,
2004 ("Reserve Agreement") (collectively with this Consent, the "Loan
Documents"); and

     WHEREAS, Current Borrower and Proposed Borrower have requested that Lender
consent to the transfer of the ownership of the Property from Current Borrower
to Proposed Borrower and Lender will consent to said transfer provided that: (i)
Proposed Borrower enters into this Consent assuming Current Borrower's
obligations under the Loan Documents; and, (ii)

                                       1

<PAGE>

Proposed Borrower assumes the obligations of Current Borrower under that certain
Environmental Indemnity Agreement dated December 20, 2004 ("Environmental
Indemnity").

     NOW, THEREFORE, in consideration of the above premises and of the benefits
to be obtained by the covenants contained herein, and for other good, valuable
and legal consideration, the receipt and sufficiency of which is hereby
acknowledged, the parties hereto intending to be legally bound hereby agree as
follows:

1.   Recitals. The Recitals hereinabove are incorporated herein by reference and
     made a part hereof. Capitalized terms used but not defined herein shall be
     defined in the Loan Documents.

2.   Lender's Consent. Lender hereby consents to the transfer of the ownership
     of the Property from Current Borrower to Proposed Borrower. Nothing herein
     shall in any way be construed to impair or affect the first lien priority
     of the Mortgage.

3.   Assumption of Loan Document Obligations. Proposed Borrower hereby agrees to
     assume all of Current Borrower's obligations under the Loan Documents and
     abide by the terms thereof. In addition, Proposed Borrower represents that
     the funds over and above the amount of the Loan used to facilitate the
     purchase of the Property are a capital contribution and are not secured,
     directly or indirectly, by an interest in the Proposed Borrower or other
     collateral assigned to the Lender.

4.   Assumption of Environmental Indemnity Obligations. Proposed Borrower hereby
     agrees to assume all of Current Borrower's obligations under the
     Environmental Indemnity as if Proposed Borrower had signed the original
     Environmental Indemnity and agrees to abide by the terms thereof.
     Notwithstanding the above, Proposed Borrower is not making any
     representations and warranties, as of the date hereof, provided in Section
     2 of the Environmental Indemnity.

5.   No Waiver. Lender's consent to the transfer of the Property and assumption
     of all of Current Borrower's obligations under the Loan Documents by
     Proposed Borrower shall not be deemed to be a waiver of Lender's
     requirements for consent to any future transfer.

6.   Transfer and Release. On the date that Proposed Borrower assumes the loan
     and acquires ownership of the Property from Current Borrower, which shall
     be the same date as the date this document and all other documents
     transferring the Property from Current Borrower to Proposed Borrower are
     recorded ("Transfer Date"), Current Borrower shall be released from all
     obligations under the Loan Documents except Current Borrower shall remain
     liable to Lender its successors and/or assigns for any environmental
     indemnity obligations specified in the Environmental Indemnity for any
     Hazardous Material (as defined in the Environmental Indemnity) introduced
     to the Property prior to the Transfer Date.

                                       2

<PAGE>

7. OFAC Compliance.

     (a)  Proposed Borrower represents and warrants that (a) Proposed Borrower
          and each person or entity owning an interest in Proposed Borrower is
          (i) not currently identified on the Specially Designated Nationals and
          Blocked Persons List maintained by the Office of Foreign Assets
          Control, Department of the Treasury ("OFAC") and/or on any other
          similar list maintained by OFAC pursuant to any authorizing statute,
          executive order or regulation (collectively, the "LIST"), and (ii) not
          a person or entity with whom a citizen of the United States is
          prohibited to engage in transactions by any trade embargo, economic
          sanction, or other prohibition of United States law, regulation, or
          Executive Order of the President of the United States, (b) none of the
          funds or other assets of Proposed Borrower constitute property of, or
          are beneficially owned, directly or to Borrower's knowledge without
          investigation, indirectly, by any Embargoed Person (as hereinafter
          defined), (c) no Embargoed Person has any interest of any nature
          whatsoever in Proposed Borrower (whether directly or to Borrower's
          knowledge without investigation, indirectly), (d) none of the funds of
          Proposed Borrower have been derived from any unlawful activity with
          the result that the investment in Proposed Borrower is prohibited by
          law or that the Agreement is in violation of law, and (e) Proposed
          Borrower has implemented procedures, and will consistently apply those
          procedures, to ensure the foregoing representations and warranties
          remain true and correct at all times. The term "EMBARGOED PERSON"
          means any person, entity or government subject to trade restrictions
          under U.S. law, including but not limited to, the International
          Emergency Economic Powers Act, 50 U.S.C. Section 1701 et seq., The
          Trading with the Enemy Act, 50 U.S.C. App. 1 et seq., and any
          Executive Orders or regulations promulgated thereunder with the result
          that the investment in Proposed Borrower is prohibited by law or
          Proposed Borrower is in violation of law.

     (b)  Proposed Borrower covenants and agrees (a) to comply with all
          requirements of law relating to money laundering, anti-terrorism,
          trade embargos and economic sanctions, now or hereafter in effect, (b)
          to promptly notify Lender in writing if any of the representations,
          warranties or covenants set forth in this paragraph or the preceding
          paragraph are no longer true or have been breached or if Proposed
          Borrower has a reasonable basis to believe that they may no longer be
          true or have been breached, (c) not to intentionally use funds from
          any "Prohibited Person" (as such term is defined in the September 24,
          2001 Executive Order Blocking Property and Prohibiting Transactions
          With Persons Who Commit, Threaten to Commit, or Support Terrorism) to
          make any payment due to Lender under the Agreement and (d) at the
          request of Lender, to provide such information as may be requested by
          Lender to determine Proposed Borrower's compliance with the terms
          hereof.

          Proposed Borrower hereby acknowledges and agrees that Proposed
          Borrower's inclusion on the List at any time during the Agreement Term
          shall be a material default of the Loan.

                                       3

<PAGE>

8.   Mortgage Modifications.

     (a)  Recital N, second paragraph, page 4, is hereby deleted.

     (b)  Permitted Transfer, page 12, subparagraphs (iv), (vi) and (vii) are
          hereby deleted.

     (c)  Specific Transfer Requirements One, Three and Four on pages 14-16 are
          hereby deleted.

     (d)  Event of Default, Section 10(f), page 24, the second paragraph is
          hereby deleted.

9.   Ownership Transfers. Lender acknowledges that the ultimate parent of the
     Proposed Borrower, Columbia Equity Trust, Inc., (the "REIT"), is a publicly
     held real estate investment trust and Lender agrees that transfers of
     direct or indirect ownership interests in the REIT or it operating
     partnership, Columbia Equity, LP (the "OP"), will not violate the
     provisions of Section 2(f) of the Mortgage, so long as the REIT continues
     to own at least 51% of the beneficial interest in the OP and remains the
     controlling general partner of the OP. Such transfers of direct or indirect
     ownership interests in the REIT and/or the OP will not require Lender
     approval nor payment of any fees to Lender.

10.  Notice. The Notice address to Borrower, as shown on the first page of the
     Mortgage and the Assignment of Leases and Rents, is hereby changed to read:
     1025 Vermont Avenue, LLC, a Virginia limited liability company, c/o
     Columbia Equity Trust, Inc., 1750 H Street, NW, Suite 500, Washington,
     D.C., 20006.

11.  Definition of Borrower. From and after the Transfer Date, wherever there is
     a reference to the "Borrower" such term shall thereafter mean and refer to
     the Proposed Borrower.

12.  Counterpart Execution. This Consent may be executed in counterparts, each
     of which shall be deemed to be an original, but all of which together shall
     constitute one and the same instrument.

13.  Ratification. Except as herein specifically modified and consented to, the
     Loan Documents shall remain in full force and effect as written and nothing
     herein shall in any way be construed to impair or affect the first lien of
     the Mortgage.

14.  General. This Consent shall be governed by the laws of the state where the
     Property is located and shall be binding upon and inure to the benefit of
     the parties hereto, their successors and assigns.

15.  Joint and Several. If more than one, all obligations of Current Borrower
     and Proposed Borrower hereunder are joint and several.

                                       4

<PAGE>

     IN WITNESS WHEREOF, Lender has caused this Consent to Transfer and
Assumption and Loan Modification to be executed under seal by Christopher J.
Henderson and Leanne S. Valentine, Second Vice President and Counsel and Second
Vice President and Counsel, respectively, of Principal Real Estate Investors,
LLC, a Delaware limited liability company, the authorized signatory of Principal
Life Insurance Company, an Iowa corporation.

                                        LENDER:

                                        PRINCIPAL LIFE INSURANCE COMPANY, an
                                        Iowa corporation

                                        By: PRINCIPAL REAL ESTATE INVESTORS,
                                            LLC, a Delaware limited liability
                                            company, its authorized signatory

ATTEST:

/s/ Donna A. Lutcavish                  By /s/ Christopher J. Henderson
-------------------------------------   ----------------------------------------
Name: Donna A. Lutcavish                Name: Christopher J. Henderson
Title: Assistant Managing Director &    Title: Second Vice President and Counsel
Secretary - Equity Closing

ATTEST:

/s/ Donna A. Lutcavish                  By /s/ Leanne S. Valentine
-------------------------------------      -------------------------------------
Name: Assistant Managing Director &     Name: Leanne S. Valentine
Title: Secretary - Equity Closing       Title: Second Vice President and Counsel

                       (Signatures Continued on Next Page)

                                       5

<PAGE>

STATE OF IOWA     )
                  )
COUNTY OF POLK    )

     On this 9th day of January, 2006, before me, the undersigned, a Notary
Public in and for the said State, personally appeared Christopher J. Henderson
and Leanne S. Valentine, to me personally known to be the identical persons
whose names are subscribed to the foregoing instrument, who being by me duly
sworn, did say that they are the Second Vice President and Counsel and Second
Vice President and Counsel, respectively, of PRINCIPAL REAL ESTATE INVESTORS,
LLC, a Delaware limited liability company, authorized signatory of PRINCIPAL
LIFE INSURANCE COMPANY, an Iowa corporation, and that the seal affixed to the
instrument is the seal of Principal Real Estate Investors, LLC; that the
instrument was signed and sealed on behalf of the corporation by Principal Real
Estate Investors, LLC, as authorized signatory of Principal Life Insurance
Company, by authority of the Board of Directors of Principal Life Insurance
Company; and that the aforesaid individuals each acknowledged the execution of
the foregoing instrument to be the voluntary act and deed of Principal Real
Estate Investors, LLC, as authorized signatories of said corporation, by it and
by them voluntarily executed.

/s/ K.L. Daniels
-------------------------------------
Notary Public in and for said State
My Commission Expires: May 2, 2008

                       (Signatures Continued on Next Page)

                                       6

<PAGE>

     IN WITNESS WHEREOF, the Trustees have signed this Consent to Transfer and
Assumption and Loan Modification as of the date and year above written.

                                        TRUSTEES:

                                        /s/ Robert L. Gorham
                                        ----------------------------------------
                                        ROBERT L. GORHAM, Trustee

                                        /s/ Joseph Whitebread
                                        ----------------------------------------
                                        JOSEPH P. WHITEBREAD, JR., Trustee

DISTRICT OF COLUMBIA, to-wit:

     The foregoing instrument was acknowledged before me this 10th day of
January, 2006, by Robert L Gorham, Trustee. My Commission expires: 12-14-2009.

                                        /s/ David M. Lautenberger
                                        ----------------------------------------
                                        NOTARY PUBLIC

DISTRICT OF COLUMBIA, to-wit:

     The foregoing instrument was acknowledged before me this 10th day of
January 2006, by Joseph P. Whitebread, Jr., Trustee. My Commission expires:
12-14-2009.

                                        /s/ David M. Lautenberger
                                        ----------------------------------------
                                        NOTARY PUBLIC

                       (Signatures Continued on Next Page)

                                       7

<PAGE>

     IN WITNESS WHEREOF, Current Borrower has caused this Consent to Transfer
and Assumption and Loan Modification to be executed as of the date and year
above written.

                                        CURRENT BORROWER:

                                        1025 VERMONT INVESTORS, L.L.C., a
                                        Delaware limited liability company

                                        By: VERMONT AVENUE MANAGING
                                            ASSOCIATES, L.L.C., a Delaware
                                            limited liability company

                                        By /s/ Andrew J. Czekaj
                                           -------------------------------------
                                        Name: Andrew J. Czekaj
                                        Title: Manager

Commonwealth of Virginia )

County of Fairfax        )

I, the undersigned and for the jurisdiction aforesaid do hereby certify that
Andrew J. Czekaj who is personally known to me as the Manager of Vermont Avenue
Managing Associates, L.L.C. the Managing Member of 1025 Vermont Avenue
Investors, L.L.C. personally appeared before me in said jurisdiction and as
attorney-in-fact as aforesaid, and acknowledged the aforesaid document to be the
act and deed of Vermont Avenue Managing Associates, L.L.C., as Managing Member
of 1025 Vermont Avenue Investors, L.L.C.

     Given under my hand and seal this 11th day of January, 2006.

                                        /s/ Therese Wolf
                                        ----------------------------------------
                                        Notary Public in and for said County and
                                        State My Commission Expires: ___________

                       (Signatures Continued on Next Page)

                                       8

<PAGE>

     IN WITNESS WHEREOF, Proposed Borrower has caused this Consent to Transfer
and Assumption and Loan Modification to be executed under seal by Oliver T.
Carr, III, the President of 1025 Vermont Avenue SPE, Inc., a Virginia
corporation, the Manager of 1025 Vermont Avenue, LLC, a Virginia limited
liability company, and hereby appoints Oliver T. Carr, III as its true and
lawful attorney-in-fact to execute and deliver this Consent to Transfer and
Assumption and Loan Modification and its act and deed, as of the date and year
above written.

                                        PROPOSED BORROWER:

WITNESS:

/s/ Celeste White                       1025 VERMONT AVENUE, LLC, a Virginia
-------------------------------------   limited liability company
Name: Celeste White

                                        By: 1025 Vermont Avenue SPE, Inc., a
                                            Virginia corporation, its manager

                                        By: /s/ Oliver T. Carr, III
                                            ------------------------------------
                                        Name: Oliver T. Carr, III
                                        Title: President

DISTRICT OF COLUMBIA, to-wit:

     The foregoing instrument was acknowledged before me this 11th day of
January, 2006, by Oliver T. Carr, III, President of 1025 Vermont Avenue SPE,
Inc., a Virginia corporation, Manager of 1025 Vermont Avenue, LLC, a Virginia
limited liability company, on behalf of said company. My Commission expires:
12-14-08.

                                        /s/ Madina M. Robinson
                                        ----------------------------------------
                                        NOTARY PUBLIC

                                       9

<PAGE>

                                    EXHIBIT A
                                     754150

ALL THAT CERTAIN lot or parcel of land situate, lying and being in the District
of Columbia, and more particularly described as follows:

Lot 47 in Square 217 in a subdivision made by Joseph J. Moebs as per plat
recorded in Liber 55 at folio 107 in the Office of the Surveyor for the District
of Columbia and being more particularly described as follows:

Beginning for the same at the southwest corner of said Lot 47, said point
marking the intersection of the southeasterly right of way line of Vermont
Avenue, N.W., 130 feet wide, and northerly right of way line of a twenty (20)
ft. wide public alley; thence with said southwesterly right of way line.

1.   North 24(Degree)31' East 123.93 feet to a point; thence

2.   Due East, 80.68 feet to a point on the westerly right of way line of a
     twenty (20) ft. wide public alley; thence with said westerly right of way
     line

3.   Due South, 112.83 feet to a point marking the intersection of the aforesaid
     westerly and northerly right of way lines of the two twenty (20) ft. wide
     public alleys; thence with aforesaid northerly right of way line of the
     twenty (20) ft. wide public alley

4.   Due West, 132.14 feet to the point of beginning containing 12,006 square
     feet or 0.2756 of an acre of land, more or less.

                                       10

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