Wis. Stat. § 402.204: Formation in general.

(1) A contract for sale of goods may be made in any manner sufficient to show agreement, including conduct by both parties which recognizes the existence of such a contract.

(2) An agreement sufficient to constitute a contract for sale may be found even though the moment of its making is undetermined.

(3) Even though one or more terms are left open a contract for sale does not fail for indefiniteness if the parties have intended to make a contract and there is a reasonably certain basis for giving an appropriate remedy.