(1) Except as provided in sub. (2), the proper money of the claim is one of the following:
(a) The money regularly used between the parties as a matter of usage or course of dealing.
(b) The money used at the time of a transaction in international trade, by trade usage, or common practice, for valuing or settling transactions in the particular commodity or service involved.
(c) The money in which the loss was ultimately felt or will be incurred by a party.
(2) The money in which the parties have contracted that a payment be made is the proper money of the claim for that payment.