<PAGE>   1

                                                                 EXHIBIT 10.23



                       LEHMAN BROTHERS REALTY CORPORATION
                            3 World Financial Center
                                200 Vesey Street
                               New York, NY 10285




                                  June 8, 1998


Arden Realty Finance IV, L.L.C.
11601 Wilshire Boulevard
Suite 402
Los Angeles, California 90025
Attention:  Diana Laing


Arden Realty Limited Partnership
Arden Realty, Inc.
11601 Wilshire Boulevard
Fourth Floor
Los Angeles, California  90025
Attention:  Diana Laing


Ladies and Gentlemen:

        Reference is hereby made to that certain mortgage loan being made by
Lehman Brothers Realty Corporation, or an affiliate thereof ("Lender") on the
date hereof in the aggregate principal amount of $100,600,000.00 (the "Loan")
pursuant to that certain Loan Agreement dated as of the date hereof between
Lender and Arden Realty Finance IV, L.L.C. ("Borrower") (the "Loan Agreement").
All capitalized terms not otherwise defined herein shall have the meanings
ascribed to them in the Loan Agreement.

        This letter agreement is between Lender, Borrower, Arden Realty, Inc.
(the "REIT") and Arden Realty Limited Partnership (the "Operating Partnership";
the REIT and the Operating Partnership hereinafter are collectively referred to
as the "Companies"). To induce Lender to make the Loan and in consideration of
Lender's making the Loan, the sufficiency of such consideration being hereby
acknowledged, the parties hereto agree as follows:

        1.     Cooperation in Securitization.



<PAGE>   2

               At the request of Lender, Borrower, the Companies and their
respective affiliates shall use reasonable efforts to satisfy the market
standards to which Lender customarily adheres or which may be reasonably
required in the marketplace or by the Rating Agencies in connection with the
sale of the Loan or participation therein or the securitization (such sale
and/or securitization, the "Securitization") of the Loan and/or the issuance of
rated single or multi-class securities (the "Securities"), including, but not
limited to:

               (a)    at Borrower's expense, (i) provide such financial and
                      other information with respect to each Mortgaged Property,
                      Borrower, the Companies and the manager of any Mortgaged
                      Property, and (ii) provide budgets relating to each
                      Mortgaged Property (the "Provided Information"), together,
                      if customary, with appropriate consents with respect to
                      and/or verification of the Provided Information through
                      letters of auditors or opinions of counsel of independent
                      attorneys reasonably acceptable to Lender and the Rating
                      Agencies:

               (b)    at Borrower's expense, assist in preparing descriptive
                      materials for presentations to any or all of the Rating
                      Agencies in connection with the Securitization (all
                      matters of form and content in respect of such materials
                      shall be subject to approval by Borrower, the Companies
                      and their respective affiliates, which shall be
                      responsible for the accuracy and completeness of such
                      materials);

               (c)    work with, and if requested, supervise, third-party
                      service providers engaged by Borrower, the Companies and
                      their respective affiliates, to obtain, collect, and
                      deliver information required by the Rating Agencies in
                      connection with the Securitization;

               (d)    if requested by Lender, execute such amendments to the
                      Loan Documents and required ancillary documents to
                      reallocate the Allocated Loan Amounts among the Mortgaged
                      Properties, provided that no such reallocation shall
                      result in the imposition of any additional mortgage
                      recording, intangibles or documentary stamp taxes, or
                      title insurance premiums or charges, and provided further
                      that no Allocated Loan Amount for any one Mortgaged
                      Property shall be increased or decreased by more than ten
                      percent (10%);

               (e)    if required by the Rating Agencies, and at Borrower's
                      expense, deliver revised opinions of counsel as to
                      non-consolidation, due execution and enforceability with
                      respect to the Mortgaged Properties, Borrower, the
                      Companies and their respective affiliates, and the Loan
                      Documents, which counsel and opinions shall be reasonably
                      satisfactory to Lender and the



                                      -2-
<PAGE>   3

                      Rating Agencies;

               (f)    if required by the Rating Agencies, and at Borrower's
                      expense, deliver such additional tenant estoppel letters
                      or other estoppels from parties to agreements that affect
                      the Mortgaged Properties, which estoppel letters shall be
                      reasonably satisfactory to Lender and the Rating Agencies;

               (g)    make such representations and warranties as of the closing
                      date of the Securitization with respect to the Mortgaged
                      Properties, Borrower, the Companies and the Loan Documents
                      as are customarily provided in securitization transactions
                      and as may be reasonably requested by Lender or the Rating
                      Agencies and consistent with the facts covered by such
                      representations and warranties as they exist on the date
                      thereof, in form and substance similar to the
                      representations and warranties made in the Loan Documents;

               (h)    execute such amendments to the Loan Documents as may be
                      requested by Lender or the Rating Agencies or otherwise to
                      effect the Securitization; provided, however, that
                      Borrower shall not be required to modify or amend any Loan
                      Documents if such modification or amendment would (i)
                      change the interest rate, the stated maturity or the
                      amortization of principal set forth in the Mortgage Note,
                      or (ii) materially modify or amend any other material or
                      economic term of the Loan except as permitted or required
                      pursuant to subparagraph 1(d) above;

               (i)    cooperate with Lender in the preparation of a private
                      placement memorandum, prospectus, including any amendment
                      or supplement to either thereof, or other disclosure
                      document (each, a "Disclosure Document") to be used by
                      Lender or any affiliate in connection with the sale or
                      securitization of the Loan or any securities representing
                      an interest in the Loan and providing indemnification
                      against securities liability with respect to information
                      provided by Borrower, the Companies or their respective
                      affiliates or disclosure about Borrower, the Companies,
                      their affiliates, the Loan and each Mortgaged Property;

               (j)    notwithstanding anything to the contrary contained in
                      subparagraph (h) above, upon Lender's request, the Loan
                      Documents shall be modified to provide for the delivery of
                      financial statements to Lender as may be required by the
                      Securities and Exchange Commission or the Rating Agencies;
                      and

               (k)    provide such other information and take such other action
                      in connection



                                      -3-
<PAGE>   4

               with the Securitization as may be reasonably requested by Lender
               or the Rating Agencies.

        2.     Costs and Expenses.

        All third-party costs and expenses and out-of-pocket expenses incurred
by Lender in connection with the Borrower's complying with requests made
hereunder shall be paid by Lender except for (i) the costs of third-party
service providers that prepared reports with respect to the Mortgaged Properties
in connection with the closing of the Loan, (ii) the fees and expenses of
counsel to Borrower or the Companies and any travel costs incurred by Borrower
or the Companies and (iii) any other costs or expenses expressly stated herein
to be borne by Borrower. Costs and expenses to be paid by Lender shall include,
but not be limited to, reasonable fees and disbursements of legal counsel,
accountants, and other professionals retained by Lender and fees and expenses
incurred for producing any offering documents or any other materials (including
travel by Lender and its agents, design, printing, photograph and document
production costs).

        3.     Indemnification.

               (a)    Borrower, the Companies and their affiliates understand
that certain of the Provided Information ("Disclosure Information") may be
included in a disclosure document (the "Disclosure Document") in connection with
the Securitization and may also be included in filings with the Securities and
Exchange Commission pursuant to the Securities Act of 1933, as amended (the
"Securities Act"), or the Securities and Exchange Act of 1934, as amended (the
"Exchange Act"), or provided or made available to investors or prospective
investors in the Securities, the Rating Agencies, and service providers relating
to the Securitization. In the event that the Disclosure Document is required to
be revised prior to the sale of all Securities, Borrower, the Companies and
their respective affiliates will cooperate with Lender in updating the
Disclosure Document by providing all current information pertaining to Borrower,
the Companies and the Mortgaged Properties as is necessary to keep the
Disclosure Document accurate and complete in all material respects.

               (b)    Borrower and the Companies agree to provide in connection
with each Disclosure Document an indemnification certificate (A) certifying that
Borrower and the Companies have carefully examined such memorandum or
prospectus, as applicable, including without limitation, the section entitled
"Description of the Mortgage Loans and Mortgaged Properties," and such section
(and any other sections reasonably requested ) does not contain any untrue
statement of a material fact or omit to state a material fact necessary in order
to make the statements made, in light of the circumstances under which they were
made, not misleading pertaining to Borrower, the Companies or the Mortgaged
Properties, (B) indemnifying Lender (and for purposes of this paragraph, Lender
shall include its officers, directors and employees), any affiliate of Lender to
whom the Loan may be assigned prior to or in connection with the Securitization
and the officers, directors and employees of such affiliate, the affiliate of
Lender



                                      -4-
<PAGE>   5

that has filed its registration statement relating to the Securitization (the
"Registration Statement"), each of its directors, each of its officers who have
signed the Registration Statement and each person or entity who controls the
affiliate within the meaning of Section 15 of the Securities Act or Section 20
of the Exchange Act (collectively, the "Lehman Group"), and Lender, each of its
directors and each person who controls Lender within the meaning of Section 15
of the Securities Act and Section 20 of the Exchange Act (collectively, the
"Underwriter Group") for any losses, claims, damages or liabilities (the
"Liabilities") to which Lender, the Lehman Group or the Underwriter Group may
become subject insofar as the Liabilities arise out of or are based upon any
untrue statement or alleged untrue statement of any material fact pertaining to
Borrower, the Companies and/or the Mortgaged Properties contained in such
sections or arise out of or are based upon the omission or alleged omission to
state therein a material fact required to be stated in such sections pertaining
to Borrower, the Companies and/or the Mortgaged Properties or necessary in order
to make the statements in such sections pertaining to Borrower, the Companies
and/or the Mortgaged Properties in light of the circumstances under which they
were made, not misleading and (C) agreeing to reimburse Lender, the Lehman Group
and the Underwriter Group for any legal or other expenses reasonably incurred by
any of them in connection with investigating or defending the Liabilities;
provided, however, that Borrower and the Companies will be liable in any such
case under clauses (B) or (C) above only to the extent that any such loss,
claim, damage or liability arises out of or is based upon any such untrue
statement or omission made therein in reliance upon and in conformity with the
Provided Information furnished to Lender by or on behalf of Borrower and the
Companies in connection with the preparation of the memorandum or prospectus or
in connection with the underwriting of the debt, including, without limitation,
financial statements of Borrower and the Companies, operating statements, rent
rolls, environmental site assessment reports and property condition reports with
respect to the Mortgaged Properties. The indemnity provided in any such
indemnification certificate shall be in addition to any liability which Borrower
and the Companies, may otherwise have.

               (c)    In connection with subsequent filings under the Exchange
Act in connection with the Securitization, Borrower, the Companies and their
affiliates agree to indemnify (i) Lender, the Lehman Group and the Underwriter
Group for Liabilities to which the Lender, the Lehman Group or the Underwriter
Group may become subject insofar as the Liabilities arise out of or are based
upon the omission or alleged omission to state in the Disclosure Information a
material fact required to be stated in the Disclosure Information in order to
make the statements in the Disclosure Information or securities filings of
Borrower or either of the Companies, in light of the circumstances under which
they were made, not misleading and (ii) reimburse Lender, the Lehman Group or
the Underwriter Group for any legal or other expenses reasonably incurred by
Lender, the Lehman Group or the Underwriter Group in connection with defending
or investigating the Liabilities; provided none of Borrower or either of the
Companies shall have any obligation to review or examine such subsequent filings
or provide any additional information or take other actions subsequent to the
closing of the



                                      -5-
<PAGE>   6

Securitization other than as required by the Loan Documents.

               (d)    Promptly after receipt by an indemnified party hereunder
of notice of the commencement of any action, such indemnified party will, if a
claim in respect thereof is to be made against the indemnifying party hereunder,
notify the indemnifying party in writing of the commencement thereof, but the
omission to so notify the indemnifying party will not relieve the indemnifying
party from any liability which the indemnifying party may have to any
indemnified party hereunder except to the extent that failure to notify causes
prejudice to the indemnifying party. In the event that any action is brought
against any indemnified party, and it notifies the indemnifying party of the
commencement thereof, the indemnifying party will be entitled, jointly with any
other indemnifying party, to participate therein and, to the extent that it (or
they) may elect by written notice delivered to the indemnified party promptly
after receiving the aforesaid notice from such indemnified party, to assume the
defense thereof with counsel satisfactory to such indemnified party. After
notice from the indemnifying party to such indemnified party hereunder, the
indemnifying party shall be responsible for any legal or other expenses
subsequently incurred by such indemnified party in connection with the defense
thereof other than reasonable costs of investigation; provided, however, if the
defendants in any such action include both the indemnified party and the
indemnifying party and the indemnifying party shall have reasonably concluded
that there are legal defenses available to it and/or other indemnified parties
that are different from or in addition to those available to the indemnifying
party, the indemnified party or parties may participate in the defense of such
action on behalf of such indemnified party or parties. The indemnifying party
shall not be liable for the expenses of more than one separate counsel unless an
indemnified party shall have reasonably concluded that there may be legal
defenses available to it that are different from or in addition to those
available to another indemnified party.

               (e)    In order to provide for just and equitable contribution in
circumstances in which the indemnification provided for hereunder is for any
reason held to be unenforceable by an indemnified party in respect of any
losses, claims, damages or liabilities (or action in respect thereof) referred
to therein which would otherwise be indemnifiable hereunder, the indemnifying
party shall contribute to the amount paid or payable by the indemnified party as
a result of such losses claims, damages or liabilities (or action in respect
thereof); provided, however, that no person guilty of fraudulent
misrepresentation (within the meaning of Section 11(f) of the Securities Act)
shall be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation. In determining the amount of contribution to which
the respective parties are entitled, the following factors shall be considered:
(i) the proportion as is appropriate to reflect the relative benefits to
Borrower and the Companies on the one hand, and Lender on the other in
connection with the transactions to which such indemnification or reimbursement
relates; or (ii) if the allocation provided by clause (i) is judicially
determined not to be permitted, in such proportion as is appropriate to reflect
not only the relative benefits referred to in clause (i) but also the relative
faults of Borrower and the Companies on the one 



                                      -6-
<PAGE>   7
hand, and Lender on the other, as well as any equitable considerations. Lender
and Borrower and the Companies herein agree that it would not be equitable if
the amount of such contribution were determined by pro rata or per capita
allocation.

               (f)    The liabilities and obligations of Borrower, the Companies
and their respective affiliates hereunder shall survive the termination of the
Loan Agreement and the satisfaction and discharge of the Debt.

               4.     Miscellaneous.

                      The terms and conditions of this letter shall survive, and
continue in full force and effect after, the Loan closing, if any, and all
references herein to the Loan Agreement and the Loan Documents shall refer to
the actual Loan Agreement and Loan Documents executed and delivered upon the
closing of the Loan.

                      The failure of Borrower, the Companies and their
respective affiliates to pay any sums when due under this letter or to comply
with any of the covenants or obligations contained in this letter shall be an
Event of Default under the Loan Agreement.

                      This letter may not be amended by any provision hereof
waived or modified except by an agreement in writing signed by each of the
parties hereto. This letter shall be governed by, and construed in accordance
with, the laws of the State of New York (but not including the choice of law
rules thereof).



                           [SIGNATURE PAGES TO FOLLOW]











                                      -7-
<PAGE>   8


ACCEPTANCE

               If the foregoing correctly sets forth Lender's understanding with
Borrower and the Companies, Borrower and the Companies should indicate their
acceptance of the terms hereof by signing in the appropriate space below and
returning to Lender the enclosed duplicate original of this letter, whereupon
this letter shall become a binding agreement among Lender, the Borrower and the
Companies.

                                        LEHMAN BROTHERS REALTY CORPORATION,
                                        a Delaware corporation


                                        By:
                                        Name:
                                        Title:



AGREED to on the 8th day of June, 1998


                                        ARDEN REALTY FINANCE IV, L.L.C.,
                                        a Delaware limited liability company


                                        By:
                                        Name:
                                        Title:



                                        ARDEN REALTY, INC.,
                                        a Maryland corporation


                                        By:
                                        Name:
                                        Title:




<PAGE>   9




                                        ARDEN REALTY LIMITED PARTNERSHIP,
                                        a Maryland limited partnership

                                        By: Arden Realty, Inc., General Partner


                                            By:
                                            Name:
                                            Title: