Mass. Gen. Laws Ann. ch. 140D, §28A
(a)(1) With respect to a credit card which may be used for extensions of credit in sales transactions in which the seller is a person other than the card issuer, the card issuer may not, by contract or otherwise, prohibit any such seller from offering a discount to a cardholder to induce the cardholder to pay cash, check or similar means rather than use a credit card.
(b) With respect to any sales transaction, any discount from the regular price offered by the seller for the purpose of inducing payment by cash, check or other means not involving the use of an open-end-credit plan or a credit card shall not constitute a finance charge under section four if such discount is offered to all prospective buyers and its availability is disclosed clearly and conspicuously.