If you want the law, here it is. If you give the countefeit currency back to the customer, you are passing counterfeit currency and would be subject to 20 years in prison:
18 USC § 472 - Uttering counterfeit obligations or securities
Whoever, with intent to defraud, passes, utters, publishes, or sells, or attempts to pass, utter, publish, or sell, or with like intent brings into the United States or keeps in possession or conceals any falsely made, forged, counterfeited, or altered obligation or other security of the United States, shall be fined under this title or imprisoned not more than 20 years, or both.
The opinions expressed are my own, take them or leave them.