Civil Code, Art. 1252

He who has various debts of the same kind in favor of one and the same creditor, may declare at the time of making the payment, to which of them the same must be applied. Unless the parties so stipulate, or when the application of payment is made by the party for whose benefit the term has been constituted, application shall not be made as to debts which are not yet due. If the debtor accepts from the creditor a receipt in which an application of the payment is made, the former cannot complain of the same, unless there is a cause for invalidating the contract. (1172a)