Section 3--119. Other Writings Affecting Instrument.

(1) As between the obligor and his immediate obligee or any transferee
the terms of an instrument may be modified or affected by any other
written agreement executed as a part of the same transaction, except
that a holder in due course is not affected by any limitation of his
rights arising out of the separate written agreement if he had no notice
of the limitation when he took the instrument.

(2) A separate agreement does not affect the negotiability of an
instrument.