counterletter

noun

1. : a secret agreement that expresses the true intent of the parties to a simulation; especially : a writing embodying the agreement counterletter can have no effects against third persons in good faith —Louisiana Civil Code : a writing embodying the agreement counterletter can have no effects against third persons in good faith —Louisiana Civil Code

   Note: An agreement stating that the conveyor of some land is still the owner, even though the other party has been given a bill of sale, is an example of a counterletter.