Someone keeps giving us fake bills. What to do?

I come into work today and my boss tells me someone gave us a fake $50 and a fake $20. They couldnt catch them on the cameras. She tells me if I get handed a fake bill then I need to call the police department. What should I do with the customer? Should I just stall them until the police department gets here? Because I'm sure theyre going to be suspicious when I don't give them their money back and my boss REALLY wants to catch them.