Girls do this crap all the time. They make promises they can’t keep and then look to others to help them craft the perfect lie / justification to help them get out of it. Guys do it too but at least they can come up with their own lies.
Just tell him / her you can’t give it to them, and tell them why. Sure, they might be mad about it, but don’t let them get too mad about it, really what did you do? They were going to get something, but now they’re not. Hard to get too mad about that.