Suppose one were to have a coupon for an upgrade or free whatever at a hotel or airline, and suppose one made a reservation using the coupon, and then checked in and actually received the full benefit of the coupon from the hotel or airline, but the check in agent failed to collect the coupon. And suppose the coupon holder then anticipated no further opportunities to use the coupon again before its expiration date.

Would it be unethical to offer said coupon to someone else, say, in this forum, considering that the hotel or airline had provided the full value in good faith, and had not issued the coupon with the intent that it be used multiple times?