Counterfeit marker

I used to work in a business where large sums of baknotes changed hands on a daily basis. It wasn't unusual to have had $20,000- $40,000 pass through your hands in an 8 hour day. 99.9% of the time, you can tell a counterfeit just by the way it feels, there's no need to even look at a bill to ascertain that it's the real McCoy. When in comes to large banknotes however, you naturally want to err on the side of caution, have a good-look and feel, and possibly use a counterfeit detection device. We had a special safe, equipped with a scanner that would reject any counterfeit bills (a Brinks "Compusafe") Ideally, you should retain a counterfeit note and hand it over to the RCMP/Secret Service/Scotland Yard or the equivalent in your jurisdiction. In this far from ideal world, however, you ALWAYS give a counterfeit banknote back to the person who handed it to you to avoid the possibility of violence. The swindler will then try to pass it to a hapless clerk elsewhere who's without the benefit of a counterfeit detector or brains. My "counterfeit stamper" would be a simple yet worthwile improvement on the safe that I've described: Prior to spitting the counterfeit bill out at you, the bill would be marked in bold letters "COUNTERFEIT - VOID" or something to the effect. The printer would consist of either an Ithaca printer (the simple printer used to print receipts and bus transfers, etc) OR there could be a self-inking stamp inside the safe.

The person with the counterfeit bill would be unable to pass the bill anywhere, he wouldn't get mad at the clerk in question, after all, the clerk didn't mark the bill, the safe did!