Fairies are among the sneakiest and least forgiving of all supernatural creatures.
It's dangerous to make a deal with any supernatural creature. The Wee Folk in particular are famous for their arrogance, touchy temper, fondness for playing pranks, and their tendency to steal anything from thimbles to fully grown human beings. This being so, humans would do well to avoid being put in the position of making any kind of deal with one or more of the Fae.
As the saying goes, good stories come from bad decisions, so folklore has more than a few tales of humans who have tried to solve a problem by making a deal with a fairy. The usual pattern of folk tales results in the fairy coming up with an interpretation of the deal's terms that makes the human's problems that much worse. Often the human's own carelessness brings about an unforseen fulfillment of the terms of the deal.
Most of the time the fairies get the best of humans, but there are occasions when the human comes out the winner. Outsmarting the Wee Folk is not a good way to make long term friends of them. Matters go two ways from there. One, the fairies respect the human and keep their distance. Two, the fairies' revenge is well timed and very costly.
Lillian's fiction has appeared in Fantastic Stories, These Vampires Don't Sparkle, and DAW'S The Year's Best Horror Stories XXI and XXII. Her Christmas ghost story "The Family Spirit" appeared in Weird Tales #322 and "Maeve" appeared in #333.
The perils of bargaining with a powerful fairy is the central theme of her short story "Beware the Fairy's Price." This story will appear in Wee Folk and Wise, a new anthology forthcoming from Sky Warrior Press. Visit Lillian at lillian888.wordpress.com.