Subject to applicable law regarding exclusion of proof of contemporaneous or prior agreements, the obligation of a party to an instrument to pay the instrument may be modified, supplemented, or nullified by a separate agreement of the obligor and a person entitled to enforce the instrument if the instrument is issued or the obligation is incurred in reliance on the agreement or as a part of the same transaction giving rise to the agreement. To the extent an obligation is modified, supplemented, or nullified by an agreement under this section, the agreement is a defense to the obligation.