E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
I will pay you in monkeys money (en monnaie 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 monkeys money.Rabelais: Gargantua and Pantagruel, iv. 3.