Cal. Civ. Code 1521-1524

1521.  An accord is an agreement to accept, in extinction of an obligation, something different from or less than that to which the person agreeing to accept is entitled.

1522.  Though the parties to an accord are bound to execute it, yet it does not extinguish the obligation until it is fully executed.

1523.  Acceptance, by the creditor, of the consideration of an accord extinguishes the obligation, and is called satisfaction.

1524.  Part performance of an obligation, either before or after a breach thereof, when expressly accepted by the creditor in writing, in satisfaction, or rendered in pursuance of an agreement in writing for that purpose, though without any new consideration, extinguishes the obligation.