Mo. Rev. Stat. § 400.3-601: Discharge and effect of discharge.

(a) The obligation of a party to pay the instrument is discharged as stated in this Article or by an act or agreement with the party which would discharge an obligation to pay money under a simple contract.

(b) Discharge of the obligation of a party is not effective against a person acquiring rights of a holder in due course of the instrument without notice of the discharge.