As another FTer has done on another thread, I'll call this assertion: I think United is entitled to stand firm in this case. If you don't want the free ticket, you're free not to accept the offer of compensation for voluntary denied boarding that day, and go ahead and get on that flight. You have the right to challenge the gate personnel (politely) and call attention to the departure management card and other documentation which seems to indicate that vouchers should still be available, but when you actually accept the free ticket, to me, that's an oral contract you've made -- tit for tat.