I will pay you in monkey's money (“en monnaic de singe”)—in goods, in personal work, in mumbling and grimace. The French had a law that when a monkey passed the Petit Pont, of Paris, if it was for sale it was to pay four deniers (two-thirds of a penny) for toll; but if it belonged to a showman and was not for sale, it should suffice if the monkey went through his tricks.
“It was an original by Master Charles Charmois, principal painter to
King Megistus [of France], paid for in court fashion with monkey's
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894