Article 12 CISG
Any provision of article 11, article 29 or Part II of this Convention that allows a contract of sale or its modification or termination by agreement or any offer, acceptance or other indication of intention to be made in any form other than in writing does not apply where any party has his place of business in a Contracting State which has made a declaration under article 96 of this Convention. The parties may not derogate from or vary the effect or this article.