On occasion someone might be in a situation where he is forced to agree to participate in an act of halakhic significance, but he tells the witnesses that he is being forced to do this against his will. Such a statement – referred to by the Gemara as a moda’a (preemptive declaration) – may be written down by the witnesses and used to prove that the action was not one that he really agreed to.
The Gemara explains that in cases where the person’s agreement is an essential part of the act – e.g. a divorce or a gift – it is obvious that such a statement should work, even with no reason or explanation given, since the individual has made clear that he is not a willing participant. A case that all agree would be one where a moda’a will be important is like the case of an orchard, where someone leased the orchard for three years. When the three years was over he said to the owner “either agree to sell me this land, or else I will destroy the lease that I am holding and claim that I purchased the land from you.” If the true owner does not have a record of the lease, this threat is a real one, since the person who leased the field will be believed in court since he can prove that he worked the field for three years with no complaint from the owner.
Rabbeinu Ḥananel explains that the true owner will have to warn witnesses beforehand that he is being forced to sell the land, and then bring them with him when he demands to have the land returned to him. The witnesses will then hear the person on the land claim to have purchased it, and when the true owner follows by agreeing to sell it to him, the witnesses will be able to record that they know that he was selling it under duress.