Yes, the giver can do this, which is one reason not to take the chance.

In addition, the airlines are onto all the tricks. They can cancel your ticket, forcing you to buy a ticket at the walk-up price or not travel, and cancel the giver's frequent flyer account, including forfeiting all miles. They don't do it often, because it takes up staff time they can use elsewhere, but they don't always let it go either.

I would strongly advise against it unless you know the giver well enough to trust him/her not to play games with you, and to answer personal questions about him/her from an airline agent when you check in. A gambler might feel differently, but airlines - like casinos - have the edge.