Chat de Beaugency
(Le). Keeping the word of promise to the ear, but breaking it to the sense. The legend is this: An architect was employed to construct a bridge over the Loire, opposite Beaugency, but not being able to accomplish it, made a league with the devil to give his sable majesty the first living being which crossed the bridge. The devil supposed it would be the architect himself, but when the bridge was finished the man threw a cat forwards, and it ran over the bridge like a wild thing. The devil was furious, but a bargain's a bargain, and the “cat of Beaugency” became a proverb.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894