Mass. Gen. Laws ch. 255B § 16
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. Such refund credit shall be computed on a method which is at least as favorable to the buyer as the actuarial method, so-called. If the prepayment is made other than on an installment due date, it shall be deemed to have been made on the first installment due date if the payment is before that date, and in any other case it shall be deemed to have been made on the next preceding or next succeeding installment due date, whichever is nearer to the date of prepayment. Where the amount of the credit for anticipation of payment is less than one dollar, no refund need be made.
Mass. Gen. Laws ch. 255B, § 16