I guess demons have no reason to lie the way humans would, but come on, that deal's a little fishy suspishy. It's in the plot summary though, so I guess she's going with it? Maybe she'll reject next chapter and then something will happen to make her agree?
I was going to say that she could have done things without legs and that arms would be more beneficial, but I thought about it a bit more, and they probably don't have elevators or accessibility ramps. Heck, maybe they don't even have wheelchairs (but that's a stretch). Who knows if they even care about accessibility, so yeah, legs would probably be better in this setting.