In Disney's 1989 animated film The Little Mermaid, Ariel trades her voice to Ursula for human legs.
Ursula offers Ariel a three-day transformation into a human. In exchange, Ariel must surrender her voice, the quality that makes it possible for her to communicate and that later helps Ursula impersonate her. The bargain is presented as a magical contract with a serious condition: Ariel must receive a kiss from Prince Eric before the three days end.
Ariel's voice is not merely removed temporarily. Ursula captures it inside the magical nautilus shell she wears, turning Ariel's desire for life on land into a source of power. Ariel can still move and interact as a human, but she cannot explain who she is or that she rescued Eric.
The bargain adapts the fairy tale's transformation theme while changing important details. In Disney's version, the traded item is Ariel's voice, not her memory, crown, or life itself.