<PAGE>   1

                                                                         EX-10AA


                            SUPPLEMENTAL BENEFIT PLAN

                                       OF

                         THE CHASE MANHATTAN BANK, N.A.

                             INSTRUMENT OF AMENDMENT


         WHEREAS, The Chase Manhattan Bank, N.A. has reserved the right to amend

its Supplemental Benefit Plan (the "Plan") and now desires to amend the Plan as

set forth in this Instrument of Amendment;


         NOW, THEREFORE, the Plan is hereby amended as follows:

     1.  Section II-A(4)(c) of the Plan is amended by inserting at the end
         thereof the following:

                  Any Approved Change in Control shall not be a
                  Change in Control hereunder. As used herein,
                  the term "Approved Change in Control" means any
                  Change in Control occurring by reason of or
                  upon the occurrence of the transactions and
                  events contemplated by the Merger Agreement.
                  "Merger Agreement" means any agreement or plan
                  of merger or consolidation between CMC and
                  Chemical Banking Corporation that is approved
                  by the Boards of Directors of CMC and Chemical
                  Banking Corporation on or before September 30,
                  1995, as modified from time to time.


     2.  Section II-A is amended by adding the following as a new subsection 5.


                  Special Change in Control Benefits. If,
                  after an Approved Change in Control, the
                  employment of an employee not otherwise

<PAGE>   2
                                       2


                  eligible to receive a benefit under this Plan
                  is terminated for any reason (other than for
                  Cause or by such employee without Good Reason)
                  (a "Protected Employee"), the benefits of such
                  Protected Employee under the Plan shall be
                  immediately 100% vested and non-forfeitable on
                  the date of any such termination of employment.


     3.  Clause (i) of the definition of "Good Reason" in Section II-A(4)(h) of
         the Plan is amended by adding the following additional proviso to the
         end thereof:


                  ; provided further, however, that no event
                  described in this clause (i) shall constitute
                  Good Reason for purposes of Section II-A(5)
                  hereof;


     4.  Section II-A(3) is amended by adding the following new sentences at the
         end thereof:


                  For purposes of this Section II-A(3), the
                  definition of "Change in Control" shall be that
                  set forth in Section II-A(4)(c), except that
                  the third sentence of Section II-A(4)(c) shall
                  be ignored. The definition of "Good Reason"
                  applicable to any Potential Change in Control
                  related to any Approved Change in Control
                  shall, for purposes of this Section II-A(3),
                  exclude subsection (i) of the definition of
                  "Good Reason" set forth in Section II-A(4)(g)
                  hereof.


     5.  Article III is amended by adding the following proviso:


                  ; provided, however, that no such action shall
                  adversely affect an individual's rights under
                  the Plan without the written consent of such
                  individual if such action is taken within 24
                  months after a Change in Control or within 12
                  months after a Potential Change in Control
                  (excluding for this purpose the third sentence
                  of Section II-A(4)(c) above).


<PAGE>   3

                                       3

     6.  The amendments set forth above shall be effective as of 
         August 25, 1995.


         IN WITNESS WHEREOF, The Chase Manhattan Bank, N.A. has caused this

Instrument of Amendment to be executed this 27th day of August, 1995.


                                                  THE CHASE MANHATTAN BANK, N.A.


                                                  By:  /s/ JOHN J. FARRELL
                                                     ---------------------------
                                                     John J. Farrell
                                                     Executive Vice President