Whenever a loan upon which interest has been collected in advance, shall be paid in full before maturity thereof, the licensee shall allow or refund to the maker or makers a sum equivalent to interest, at the same rate as charged on the loan, calculated on the total amount of the unpaid installments or any evidence or certificate of indebtedness or investment hypothecated on such loan, anticipated for the period from the date of such payment to the maturity of the last installment so anticipated.