<PAGE>   1

                                                                   EXHIBIT 10.40

                                    FBCC CO.
                         500 CRESCENT COURT, SUITE 300
                              DALLAS, TEXAS 75201


                               November 10, 1998

AMC, Inc.
Suite 2200
240 Peachtree Street, N.W.
Atlanta, GA 30303

EC Holdings, Inc.
Suite 2200
240 Peachtree Street, N.W.
Atlanta, GA 30303

Re:  Class A Elevated Antecedent Debt Notes due July 31, 2000

Gentlemen:

FBCC has received a form of consent letter (the "Consent Letter") from Reliance
Trust Company (the "Trustee") dated October 29, 1998, requesting that FBCC Co.
("FBCC") consent to a substitution of a part of the collateral (the "EC 
Collateral") that secures the Class A Elevated Antecedent Debt Notes due July 
31, 2000 (the "Series C Notes") issued by AMC, Inc., a Georgia corporation 
("AMC"). The EC Collateral consists of partnerships interests in and the right 
to receive distributions from Embarcadero Center Investors Partnership, Two 
Embarcadero Center West, and Three Embarcadero Center West, each a California 
limited partnership (collectively, the "EC Partnerships"). The EC Collateral is 
owned by a wholly-owned subsidiary of AMC, EC Holdings, Inc., a Georgia 
corporation ("ECH"), and has been pledged by ECH to partially secure the 
obligation of AMC to pay the Series C Notes.

The Consent Letter requests that FBCC agree to a substitution of the EC 
Collateral for units of Boston Properties Limited Partnership, a Delaware 
limited partnership ("BPLP"). Those units are known as "Series Two Preferred 
Units" (the "BPLP Units") and are more particularly described in the BPLP 
Certificate of Designations ("Certification of Designations") Establishing and 
Fixing the Rights, Limitations and Preferences of a Series of Preferred Units 
to be executed by Boston Properties, Inc., as general partner of BPLP. The 
substitution of collateral is necessary to facilitate a proposed sale by the EC 
Partnerships of all of their assets to BPLP pursuant to the Master Transaction 
Agreement entered into among BPLP, ECH, the EC Partnerships et al. (the "Master 
Agreement").

This letter sets forth the terms and conditions by which (i) FBCC will deliver
its unconditional consent to a substitution of the EC Collateral for the BPLP
Units and (ii) AMC and ECH will redeem, or cause to be redeemed, all of the
Series C Notes. For and in consideration of the mutual covenants and agreements
set forth in this letter, the sufficiency of which are hereby acknowledged,
FBCC, AMC, and ECH hereby agree as follows:

<PAGE>   2

November 10, 1998
Page 2

1.   UNCONDITIONAL DELIVERY OF CONSENT. Upon AMC and ECH executing and
     delivering to FBCC a counterpart of this letter, FBCC will countersign the
     Consent Letter in the form attached hereto as Exhibit A and immediately
     deliver the same to the Trustee.

2.   SUBSTITUTION OF COLLATERAL. AMC and ECH will cooperate in good faith with 
     the Trustee to consummate the substitution of the EC Collateral for the 
     BPLP Units as soon as reasonably practicable, pursuant to the terms and 
     conditions set forth in the Certification of Designations and the Master 
     Agreement. The parties acknowledge that the closing of the above 
     collateral exchange is dependent upon the closing of all of the 
     transactions contemplated by the Master Agreement, over which neither of 
     the parties hereto have control. Neither AMC nor ECH will agree to any 
     material change, modification, or amendment of any of the terms and 
     conditions of the Certification of Designations and the Master Agreement 
     without the prior written consent of FBCC. AMC and ECH will periodically 
     advise FBCC, but no less frequently than once per week, as to the status 
     of the substitution of the EC Collateral for the BPLP Units. As soon as 
     reasonably practicable, but in any event by December 31, 1998, ECH and AMC
     will furnish to FBCC final copies of the Master Agreement, BPLP 
     Partnership Agreement, the Registration Rights and Lock-Up Agreement. 
     Certificate of Designations, and any other documents affecting the 
     ownership or disposition of the BPLP Units.

3.   REDEMPTION. Promptly after the substitution of the EC Collateral for the 
     BPLP Units, but in no event later than thirty (30) business days 
     thereafter, FBCC will purchase from ECH for cash and in redemption of 
     Series C Notes held by FBCC a sufficient number of BPLP Units to permit 
     AMC to redeem all of the issued and outstanding Series C Notes. ECH and 
     AMC have obtained the consent of all necessary persons, including 
     specifically the consent of the General partner of BPLP, to effectuate and 
     consummate the transfer of the BPLP Units to FBCC (and the registration of 
     the BPLP Units in the name of FBCC) as contemplated by this paragraph. No 
     consent of any other person is required to consummate the transfer of the 
     BPLP Units to FBCC as contemplated by this paragraph.

     As of the date hereof, $7,998,840.36 principal amount of the Series C 
     Notes are issued and outstanding and, therefore, a total aggregate amount 
     of $7,998,840.36 in cash would be required to redeem all of the Series C 
     Notes, including Series C Notes held by FBCC. As of the date hereof, FBCC 
     owns $2,908,689.13 principal amount of the Series C Notes. The BPLP Units 
     each have a value of $50.00 and ECH anticipates receiving approximately 
     600,000 BPLP Units pursuant to the Master Agreement. In order to effect 
     the redemption of the Series C Notes, ECH will transfer to FBCC 177,762 
     BPLP Units, having an aggregate face value of $8,887,600.40, in 
     consideration of which FBCC will surrender to AMC as fully redeemed at par 
     its $2,908,669.13 of Series C Notes and will pay to ECH in cash the sum of 
     $5,090,171.38. AMC will simultaneously redeem for cash all of the 
     remaining Series C Notes at par; provided, however, that in the event FBCC 
     acquires additional Series C Notes prior to such 
<PAGE>   3
November 10, 1998
Page 3


     redemption, then such additional Series C Notes so acquired by FBCC will be
     exchanged for additional BPLP Units at the same rate of exchange as
     provided for in this paragraph. All accrued and unpaid distributions
     payable by BPLP with respect to the BPLP Units to be assigned to FBCC will
     inure to the benefit of FBCC from and after the date of initial issuance of
     the BPLP Units by BPLP. FBCC will take the BPLP Units subject to all the
     terms and conditions of, and benefits accruing to BPLP Units, as described
     in the Certificate of Designations, the Registration Rights and Lock-Up
     Agreement and the partnership agreement of BPLP. The Lock-Up Period for the
     BPLP Units is one (1) year from the date of execution of the Registration
     Rights and Lock-Up Agreement.

4.   MISCELLANEOUS.

     a.  Entire Agreement.  This Agreement contains the entire agreement between
         the parties, and no promise, representation, warranty, or covenant not
         included in this Agreement or any such referenced agreements has been
         or is relied upon by the parties hereto.

     b.  Liability.  EC Holdings, Inc, and AMC are jointly and severally liable
         for the obligations under this Agreement.

     c.  Amendments.  No modification or amendment of this Agreement is of any
         force or effect unless made in writing and executed by the parties
         hereto.

     d.  Time.  Time is of the essence. If the exchange of the EC Collateral for
         the BPLP Units is not consummated by December 31, 1998, the Consent
         Letter shall be deemed null and void and of no further force and
         effect.

     e.  Jurisdiction.  This Agreement will be construed under and in accordance
         with the laws of the State of Texas, and all obligations of the parties
         created hereunder are performable in Dallas County, Texas.

     f.  Attorney's Fees.  Any party to this Agreement who is the prevailing
         party in any legal proceeding against the other party brought under or
         with respect to this Agreement will be additionally entitled to recover
         court costs and reasonable attorneys' fees from the non-prevailing
         party.

     g.  Counterparts.  This Agreement may be executed in any number of
         counterparts, each of which is an original, and all of which taken
         together constitute one and the same instrument.

     h.  Authority and Further Acts.  This Agreement has been duly authorized
         and delivered, and is binding on, both ECH and AMC and is enforceable
         in accordance with its terms. In addition to the acts required of the
         parties to this Agreement, the parties hereto will take any and all
         such further acts as

<PAGE>   4

November 10, 1998
Page 4


may be reasonably necessary to consummate the transactions contemplated hereby.


                     REMAINDER OF PAGE INTENTIONALLY BLANK
                            SIGNATURE PAGE FOLLOWS.
<PAGE>   5

November 10, 1998
Page 5


If you agree to be bound by the terms of this Agreement, then please have the
necessary parties execute where indicated below.

Very truly yours,
FBCC CO., a Texas general partnership


By:  /s/ PAUL E. ROWSEY, JR.
     -------------------------------
     Name: Paul E. Rowsey, Jr.
     Title: Agent


AGREED TO AND ACCEPTED this
10th day of November, 1998:

EC HOLDINGS, INC.

By:  /s/ Henry G. Almquist, Jr.
     ------------------------------
     Name: Henry G. Almquist, Jr.
     Title: Treasurer


AGREED TO AND ACCEPTED this
10th day of November, 1998:

AMC, INC.

By:  /s/ Henry G. Almquist, Jr.
     ------------------------------
     Name: Henry G. Almquist, Jr.
     Title: Chief Financial Officer



                       SIGNATURE PAGE TO LETTER AGREEMENT