"Final payment" occurs when a payee acquires ownership of the money paid, so that payment can no longer be revoked by the payor or recovered by self-help. But final payment in this sense is not the end of the story, because a person who has made a payment as a result of fraud or mistake has a prima facie claim in restitution to get the money back. "Final payment" is therefore the point at which restitution begins. Finality in a different sense—meaning the point at which a payee is protected from a liability in restitution—is determined by standard affirmative defenses, most notably the rule of Price v. Neal. Confusion between the two kinds of finality distorts payments law and leads to errors—either too much restitution or not enough—in a number of settings, some of them diverting.
Restitution and Final Payment,
Chi.-Kent L. Rev.
Available at: https://scholarship.kentlaw.iit.edu/cklawreview/vol83/iss2/8