term:

Counterletter

counterletter n

in the civil law of Louisiana
: a secret agreement that expresses the true intent of the parties to a simulation
;esp
: a writing embodying the agreement [ 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.

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.