Ringing the Changes
A method of swindling by changing gold and silver in payment of goods. For example: A man goes to a tavern and asks for two-pennyworth of whisky. He lays on the counter half a sovereign, and receives nine shillings and tenpence in change. “Oh!” (says the man) “give me the half-sovereign back, I have such a lot of change.” He then takes up ten shillings in silver and receives back the half-sovereign. The barmaid is about to take up the silver when the man says, “Give me a sovereign in lieu of this half-sovereign and ten shillingsworth of silver.” This is done, and, of course, the barmaid loses ten shillings by the transaction.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894