Principles of International Commercial Contracts, 1994 - UNIDROIT
Chapter 3 - Validity
Article 3.13 - Loss of Right to Avoid
(1) If a party is entitled to avoid the contract for mistake but the other party declares itself willing to perform or performs the contract as it was understood by the party entitled to avoidance, the contract is considered to have been concluded as the latter party understood it. The other party must make such a declaration or render such performance promptly after having been informed of the manner in which the party entitled to avoidance had understood the contract and before that party has acted in reliance on a notice of avoidance.
(2) After such a declaration or performance the right to avoidance is lost and any earlier notice of avoidance is ineffective.