Loaning magic abilities?

I'm trying to set up a situation where a magical ability can be "loaned" to a hero (in a story, not a game, but same difference). The person who has the magic is not willing to take the risk of coming along adventuring and getting hit, but will lend them an item that will give them the temporary use of a magical ability. And I don't want the hero to get to keep the magical ability, as it's far too useful! Multiple uses for this plot, yes, but not after that.

Any thoughts on how I do this? My best guess so far is a fetish with a friendly spirit that can be "persuaded" to help someone else instead of the usual owner for a bit.