Md. Code, Commercial Law § 13–306

(a) If a gift or other inducement offered to a customer in exchange for business is not available at the time the customer complies with the conditions attached to the offer, the offeror shall:

(1) Give the customer a certificate which states in unequivocal language that:

(i) The customer has complied with the conditions which entitle him to receive the gift or other inducement; and

(ii) The offeror will give the customer an identical or substantially similar item of equal value on presentment of the certificate; and

(2) Fully comply with the terms of the certificate.

(b) The certificate may designate a period of not less than 90 days after the date the customer receives the certificate within which the certificate must be presented for redemption.