Magical Bargains - Ideas?

[KOFFEYKID]

I was trying to come up with some realistic magical bargains, and here is what I got:

Selling a magical cure for X Malady in exchange 10 years of that person's youth.
Selling aid in exchange for a portion/all of the buyers magical ability.
Bargains for a "take my death" card in exchange for saving a life.

Any others?

[Esoteric]

There's always ye olde life debt. Save someone's life and they're forever indebted to you (in most cases until they save yours at least).

[Wyrdrune]

there could be "protect that childe at all costs, she will be important someday... " side of the bargain, where the character has to dedicate his life protecting someone or something until its destiny is fulfilled.

[Slife]

Every time you call upon my power, you have to break one of your bones.

[Deadmanwalking]

I will give you magic and power in exchange for your firstborn child.

Slay my foe, the Winter Lord X, and I shall bestow great power upon thee.

Superhuman strength and speed are yours for the low price of your (entirely untrained) ability to work magic.