La. Civ. Code art. 1868
When the parties have made no imputation, payment must be imputed to the debt that is already due.
If several debts are due, payment must be imputed to the debt that bears interest.
If all, or none, of the debts that are due bear interest, payment must be imputed to the debt that is secured.
If several unsecured debts bear interest, payment must be imputed to the debt that, because of the rate of interest, is most burdensome to the obligor.
If several secured debts bear no interest, payment must be imputed to the debt that, because of the nature of the security, is most burdensome to the obligor.
If the obligor had the same interest in paying all debts, payment must be imputed to the debt that became due first.
If all debts are of the same nature and became due at the same time, payment must be proportionally imputed to all.
La. C.C. § 1868