A flight is overbooked and you're the first to volunteer. They offer a Free Ticket and you accept. Later they find they need more volunteers and up the offer... to (whatever). Do you get the original amount you agreed to, or do they figure you'd get pretty pissed being the one to volunteer and get screwed because of it.

I figure you have to give everyone the same amount. If I talked to someone else and found out they got twice as much, I'd be pretty mad. Anyone know the airlines' practices?