Payer en monnaie de singe, "to fob somebody off"... well, a pretty famous French expression! But do you know it comes from the Petit-Pont in Paris? We learn that in the book Histoire de Paris et de ses monuments (J.-A. Dulaure, 1846).
In the 13th century, king of France saint Louis received a toll right on this bridge. A toll from those who used the bridge, including animals. Especially monkeys: 4 deniers (penny, a medieval coin) each monkey! Merchants came in Paris to sell them.
And they sold a lot, because at that time, rich people wanted their own little monkeys... But if the monkey belonged to a jester, it was different. The owner could get past the toll free of charge if he did a show, with his creature. They paid en monnaie de singe, literally «with monkey’s money!