CCP §676.4: (a) 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.

(a) 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. (b) If the parties to a transaction have not otherwise agreed, the proper money of the claim, as in each case may be appropriate, is one of the following: (1) The money regularly used between the parties as a matter of usage or course of dealing. (2) 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. (3) The money in which the loss was ultimately felt or will be incurred by the party claimant.