Section 2-205 of the Uniform Commercial Code pro vides an exception to the rule that an offeror is the "master of the offer" or has a right to revoke the offer at any time. It provides that "an offer given by a merchant in a signed writing which gives assurance that it will be held open is not revocable, but in no event may such period of irrevocability exceed three months." The CISG's firm offer rule is found in Article