<PAGE>   1
                                                                   EXHIBIT 10.20

                     FIRST AMENDMENT TO COMMERCIAL PROPERTY
                              MANAGEMENT AGREEMENT


        This FIRST AMENDMENT TO COMMERCIAL PROPERTY MANAGEMENT AGREEMENT (the
"First Amendment") is made and entered into as of the 6th day of April, 2000, by
and between EBS BUILDING, L.L.C., a Delaware limited liability company
("Owner"), and INSIGNIA/ESG, INC., a Delaware corporation ("Manager").

                                   WITNESSETH:

        WHEREAS, Owner and Manager entered into a certain Commercial Property
Management and Leasing Agreement dated December 31, 1997 (the "Agreement") for
that certain real estate project owned by Owner and improved with buildings and
related parking facilities and common areas (as more fully described therein,
the "Project") commonly known as The Edison Building and located in the City of
St. Louis, Missouri; and

        WHEREAS, Owner has asked to terminate the Agreement effective
immediately with respect to Manager's leasing and marketing rights and
obligations relating to the Project, but otherwise has asked to retain Manager
in connection with the management of the Project, and Manager is agreeable with
such modifications to the relationship of the parties under the Agreement, in
connection with which Manager is willing to waive any sixty (60) day notice
period with respect to cancellation under Section 2.2(d) of the Agreement; and

        WHEREAS, words and phrases having defined meanings in the Agreement
shall have the same respective meanings when used herein, unless otherwise
expressly defined herein; and

        NOW, THEREFORE, in consideration of the foregoing and other
good and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree, effective as of the date hereof, as
follows:

        1. The title of the Agreement and all references to the
Agreement are hereby modified to delete the words "AND LEASING" from the same,
so that the same shall be entitled "Commercial Property Management Agreement."

        2. Section 1.1 of the Agreement is hereby amended in its entirety to
read as follows:

           1.1 Appointment. Owner hereby appoints Manager as the manager of the
        Project and Manager hereby accepts such appointment upon the terms,
        covenants and conditions set forth herein. Manager agrees to use
        reasonable efforts and to cooperate with Owner's leasing agent for the
        Project, all at no additional cost to Owner.




<PAGE>   2


        3. No cancellation fee under Section 2.2(d) of the Agreement shall be
payable in connection with this First Amendment and the termination of the
Agreement with respect to Manager's leasing and marketing rights and obligations
relating to the Project.

        4. No cancellation fee under Section 2.3(a) of the Agreement shall be
payable in connection with this First Amendment and the termination of the
Agreement with respect to Manager's leasing and marketing rights and obligations
relating to the Project.

        5. A new Section 2.6 is hereby added to the Agreement to read as
follows:

        2.6 Termination of Leasing Obligations. Manager shall have no leasing
     and/or marketing rights and shall have no claims as broker with respect to
     the Project from and after the date hereof, except only with respect to
     those persons and entities whose lease proposals for a portion of the
     Project have been approved by Owner prior to the date hereof ("Manager
     Contacts"), which Manager Contact(s), as well as the terms, conditions and
     commission to be paid in the event a portion of the Project is leased to
     said Manager Contact(s), are detailed on the schedule attached hereto as
     EXHIBIT "1" and incorporated herein by reference.

        6. Section 4.2 of the Agreement is hereby deleted in its entirety.

        7. The language of clause (a) of Section 9.2 of the Agreement is hereby
amended by deleting the reference to "Exhibit `D'" and replacing the same with a
reference to "Exhibit `1'".

        8. The first grammatical sentence of Section 9.4 is hereby amended in
its entirety to read as follows:

        "During the term of this Agreement, Owner shall make available to
     Manager finished office space (the "Management Office") on a rent-free
     basis in an amount not to exceed 3,000 square feet, and sufficient to allow
     Manager to provide first class management and construction supervision
     activities for the benefit of the Project."

        9. "EXHIBIT D" to the Agreement is hereby deleted in its entirety.

        10. Miscellaneous.

        a.  This First Amendment may be executed in one or more counterparts,
each of which shall be deemed an original and all such counterparts, taken
together, shall constitute but one and the same instrument. Facsimile signatures
on any counterpart shall be effective as an original signature, but the parties
hereto agree to deliver to the other original signatures within thirty (30) days
after the date of this First Amendment.




                                       2


<PAGE>   3


        b. Except as expressly amended and modified hereby, all of the terms and
provisions of this Agreement shall remain unchanged and in full force and effect
and are hereby ratified and confirmed.

        c. This First Amendment shall be binding on, and shall inure to the
benefit of, the parties hereto and their respective successors and assigns.

        IN WITNESS WHEREOF, Owner and Manager have respectively signed
this First Amendment to Commercial Property Management Agreement dated and
effective as of the day and year first written above.

OWNER:                                                    MANAGER:

EBS Building, L.L.C.,                                     Insignia/ESG, Inc.
a Delaware limited liability company                      a Delaware corporation

By:   PricewaterhouseCoopers L.L.P.,                      By:/s/  J. John Reis
       its Manager                                           -------------------
                                                             Regional Director

       By:/s/ Keith F. Cooper
          -------------------
             Partner





                                       3




<PAGE>   4


                                   EXHIBIT "1"
                 MANAGER CONTACTS & COMMISSION PAYMENT SCHEDULE

A.      MANAGER ONLY; CO-BROKER NOT INVOLVED:

        DEAL TYPE - NEW LEASE (AS DESIGNATED IN THE CHART ON THE FOLLOWING
        PAGE): Manager shall be paid a commission of 5% of gross rentals to be
        paid by the Manager Contact during years 1 - 5 of the initial term of
        the lease and 3% thereafter.

        DEAL TYPE - RENEWAL (AS DESIGNATED IN THE CHART ON THE FOLLOWING PAGE):
        Manager shall be paid a commission of 2% of gross rentals to be paid by
        the Manager Contact during any renewal term(s) of the lease. This
        provision does not relate to renewals of any new lease entered into with
        a Manager Contact.

B.      MANAGER WITH CO-BROKER INVOLVED:

        DEAL TYPE - NEW LEASE (AS DESIGNATED IN THE CHART ON THE FOLLOWING
        PAGE): Manager shall be paid a commission of 2.5% of gross rentals to be
        paid by the Manager Contact during years 1 - 5 of the initial term of
        the lease and 1.5% thereafter. In addition, the Co-Broker shall be paid
        at the rate of 3.5% for years 1 - 5 and 1.5% thereafter.

        DEAL TYPE - RENEWAL (AS DESIGNATED IN THE CHART ON THE FOLLOWING PAGE):
        Manager shall be paid a commission of 2% of gross rentals to be paid by
        the Manager Contact during any renewal term(s) of the lease. Any fees to
        be paid to a Co-Broker for a renewal term shall require the prior
        approval of Owner on a case-by-case basis. This provision does not
        relate to renewals of any new lease entered into with a Manager Contact.

The term "gross rentals" as used in this Exhibit "1" shall mean and refer to the
total of any annual base rent or fixed rent payable by a Manager Contact under
any lease for the initial lease term or renewal lease term, as the case may be.

A commission will be considered earned if, at any time during the 90-day period
following the date of this First Amendment (the "Initial Period"), Owner (but
not any successor in interest to Owner) enters into a lease for vacant space in
the Project with a Manager Contact as set forth below. In addition, a commission
will be considered earned if at any time during the period commencing ninety-one
(91) days and ending one hundred eighty (180) days following the date of this
First Amendment, Owner (but not any successor in interest to Owner) enters into
a lease for vacant space in the Project with a Manager Contact, provided that
(a) the principal terms of such lease have been previously agreed to in writing
by the Owner and Manager Contact prior to the end of the Initial Period, and (b)
the terms of the lease entered into substantially conform to the terms agreed
upon during the Initial Period.

Commissions shall be paid one-half upon execution of a new lease agreement, and
one-half upon occupancy by the Manager Contact. Commissions for expansions of
space and/or renewal terms shall be paid in full upon execution of the
appropriate documentation.




                                       4



<PAGE>   5

                               ONE FINANCIAL PLAZA
                                MANAGER CONTACTS

<TABLE>
<CAPTION>
------------------------------------------------------------------------------------------------------------
           MANAGER CONTACT                                  ADDRESS                             DEAL
                                                                                                TYPE
------------------------------------------------------------------------------------------------------------
<S>                                     <C>                                               <C>
Inflow                                  1860 Lincoln Street, Dallas, TX                   New
------------------------------------------------------------------------------------------------------------
Anheuser Busch                          701 Market Street, St. Louis, MO                  New
------------------------------------------------------------------------------------------------------------
Evans & Dixon                           100 N. Broadway, St. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
Energizer, Inc.                         Ralston Purina Campus, St. Louis, MO              New
------------------------------------------------------------------------------------------------------------
SW Bell                                 1010 Pine Street, S. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
Habanero Computing Solutions, Inc.      3115 S. Grand, St. Louis, MO                      New
------------------------------------------------------------------------------------------------------------
Sandberg, Phoenix, von Gontard, PC      515 N. Sixth St., St. Louis, MO                   New
------------------------------------------------------------------------------------------------------------
Lewis & Rice                            500 N. Broadway                                   New
------------------------------------------------------------------------------------------------------------
Trilinear Corp                          712 N. Second St., St. Louis, MO                  New
------------------------------------------------------------------------------------------------------------
CCMSI                                   133 S. 11th Street, St. Louis, MO                 New
------------------------------------------------------------------------------------------------------------
White Coleman                           500 Washington Ave., St. Louis, MO                Renewal
------------------------------------------------------------------------------------------------------------
The Witan Company, LLC                  500 Washington Ave., St. Louis, MO                Renewal
------------------------------------------------------------------------------------------------------------
Daniel Henry Insurance                  2350 Market Street, St. Louis, MO                 New
------------------------------------------------------------------------------------------------------------
Secure Networks                         1008 New Hampshire, Kansas City, MO               New
------------------------------------------------------------------------------------------------------------
Goldstein & Price                       100 N. Broadway, St. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
Peavey Barge                            100 N. Broadway, St. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
Michael Fox, Inc.                       500 Washington Ave., St. Louis, MO                Renewal
------------------------------------------------------------------------------------------------------------
Husch & Eppenberger                     100 N. Broadway, St. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
</TABLE>





                                       5