RCW 6.44.040: Determining money of the claim.

(1) The money in which the parties to a transaction have agreed that payment is to be made is the proper money of the claim for payment.

(2) If the parties to a transaction have not otherwise agreed, the proper money of the claim, as in each case may be appropriate, is the money:

(a) Regularly used between the parties as a matter of usage or course of dealing;

(b) 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

(c) In which the loss was ultimately felt or will be incurred by the party claimant.