I was told by a Secret Service agent years ago that the creation of an image that could be construed to be legal tender was a violation of the law. At the time we had been asked to print an advertisement that looked like a dollar bill on one side but had an ad for a chiropractor on the other. The client, the chiropractor, wanted to leave these on the floor of various places and when people bent over to pick them up they would see that they were not real and had a does your back hurt? message on them. We were told we could be arrested for counterfeiting if we printed them.
That doesnt surprise me. My understanding is that if you are going to use a likeness of U.S. currency in an advertisement, it has to be an obvious caricature that would never be mistaken for the real thing.