A man designed a device into which you can insert colored paper, and then get the ideal 100$. Experts can not distinguish them from the real ones. How does he do that, and why does he sell this device?
The man sells this device to the thief, convincing him that it prints counterfeit dollars that are indistinguishable from the real ones. He demonstrates the machine by inserting green paper into it. But green paper - this is actually real 100$, but richly painted in green. The machine simply removes green paint from them.