“To pay in monkey’s money” (French “en monnaie de singe”)
In goods, in personal work, in mumbling and grimace. In Paris when a monkey passed the Petit Pont, if it was for sale four deniers’ toll had to be paid; but if it belonged to a showman and was not for sale, it sufficed if the monkey went through his tricks.
“… being an original by Master Charles Charmois, principal painter to King Megistus, paid for in court fashion with monkey’s money.” (Rabelais, IV, iii.)
-from the always delightful Brewer’s Dictionary of Phrase and Fable