(a) Except as provided in subsection (b) hereof, the proper money of the claim is, as in each case may be appropriate, the money:
(1) regularly used between the parties as a matter of usage or course of dealing; or
(2) 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; or
(3) in which the loss was ultimately felt or will be incurred by a party.
(b) The money in which the parties have contracted that a payment be made is the proper money of the claim for that payment.