A cushion of credit account shall be automatically established by RUS for each borrower who makes a payment after October 1, 1987, in excess of amounts then due on an RUS note. Such account will bear interest at a rate of 5 percent per annum. All payments on RUS notes which are in excess of required payments and not otherwise designated shall be deposited in the borrowers' respective cushion of credit accounts. Payments received in the month in which an installment is due will be applied to the installment due. However, if the regular installment payment is received at a later date in the month, the first payment received will be applied retroactively to a cushion of credit account and the second will be applied to the installment due.