Rumpelstiltskin

In order to make himself appear more important, a miller lied and said that his daughter could spin straw into gold. The king heard of this and called for the girl, shut her in a tower with straw and a spinning wheel, and demanded that she spin the straw into gold by morning, for three nights, or be executed. She had given up all hope, when a gnomish creature appeared in the room and spun straw into gold for her in return for her necklace, then again the following night for her ring. On the third night, when she had nothing with which to reward him, the strange creature spun straw into gold for a promise that the girl's first-born child would become his.

The king was so impressed that he married the miller's daughter, but when their first child was born, the dwarf returned to claim his payment. The queen was frightened and offered him all the wealth she had if she could keep the child. The dwarf refused but finally agreed to give up his claim to the child if the queen could guess his name in three days. At first she failed, but before the final night, her messenger discovered the dwarf's remote mountain cottage and, unseen, overheard the dwarf hopping about his fire and singing.

When the dwarf came to the queen on the third day, she revealed his name - Rumpelstiltskin. And so the little man lost his bargain.