Credit upon anticipation of payments.

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 after first deducting from such finance charge an acquisition cost of $25, as the sum of the monthly balances after the month in which prepayment is made, bears to the sum of all the monthly balances under the schedule of payments in the contract. Where the amount of credit is less than $1 no refund need be made.