The debtor shall have the right to prepay at any time, without premium or fee, the entire indebtedness or any part thereof not less than the amount of one installment, or $100, whichever is less. Any prepayment in full of the indebtedness shall be credited on the date received, and no interest may be charged thereafter. Any partial prepayment made on other than an installment due date need not be credited until the next following installment due date or 30 days after such prepayment, whichever is earlier. The holder and the debtor may agree at any time that any prepayment not previously applied in satisfaction of matured installments shall be reapplied for the purpose of curing or preventing any subsequent default.