Notwithstanding the provisions of any retail installment contract to the contrary, any buyer may pay in full, at any time before maturity, the debt of any retail installment contract and in so paying such debt shall receive a refund credit thereon for such anticipation of payments. The amount of such refund shall represent at least as great a proportion of the finance charge as the sum of the monthly time balances, beginning one month after prepayment is made, bears to the sum of all the monthly time balances under the schedule of payments in the contract (commonly referred to as the rule of 78S [78’s]). If the charge as so computed is less than twenty-five dollars ($25.00), then a maximum charge of twenty-five dollars ($25.00) and no more may be retained. Where the amount of credit is less than one dollar ($1.00), no refund need be made.