We called secret service. Three weeks later they didn't come out for it.
For future reference, in suspected cases of counterfeiting it is best to call the local police/sheriff's department. The Secret Service doesn't usually get involved in point-of-sales incidents unless it is already an active investigation...
The threat of counterfeit U.S. currency to the financial system of the United States has grown in recent years. Advances in technology, the availability of scanning and printing devices and the adoption of the U.S. dollar by nations as their legal tender have exacerbated the threat. To counter these threats, the Secret Service focuses on strategic international investigations targeting counterfeiters and their distribution networks. The agency has also initiated a comprehensive international forensic counterfeit detection training program for bankers and law enforcement officers overseas. (Source)