counterletter

Main Entry: counterletter
Function: noun
in the civil law of Louisiana : 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>
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.