Mo. Rev. Stat. § 400.3-117: Other agreements affecting instrument.

Subject to applicable law regarding exclusion of proof of contemporaneous or previous 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 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.