Article 428 of Civil Code

1 - Set-off shall occur only if invoked by the person entitled to it, and may not be waived before the right to it is established. 2 - Set-off results in the extinction of the two debts to the extent of the lesser of them, from the time they become eligible for set-off. 3 - If the debtor has multiple debts, the allocation of set-off among them shall be made in the same manner as allocation upon their payment.