Cal. Civ. Proc. Code § 676.3

(a) The effect of this title may be varied by agreement of the parties made before or after commencement of an action or distribution proceeding or the entry of judgment.
(b) Parties to a transaction may agree upon the money to be used in a transaction giving rise to a foreign-money claim and may agree to use different moneys for different aspects of the transaction. Stating the price in a foreign money for one aspect of a transaction does not alone require the use of that money for other aspects of the transaction.