(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.
New York Legal Code