If a loan contract made under Section 18636 is prepaid in full by cash, a new loan, refinancing or otherwise before the final installment date, the borrower shall receive a rebate of the portion of the precomputed charge applicable to the full installment periods following the installment date nearest the date of such prepayment; provided, however, that if prepayment in full occurs on or before the third installment date the rebate shall be the difference between the total precomputed charge and the charges at the contract rate computed on unpaid principal balances by applying each payment first to charges and the remainder to principal. After the third installment date,
any prepayment made on or before the 15th day following an installment date shall be deemed to have been made on the installment date preceding such prepayment. The tender, by the borrower or at his request, of an amount equal to the unpaid balance less the required rebate must be accepted by the company in full payment of the loan contract.