I contacted the police to find out what should be done.
When you receive a fake note you should contact the police immediately so they can come by to question the person as to where he may have received it.
If the person is gone then the police should be contacted anyway to make an "Anzeige" (report) against an unknown person.
If they catch somebody making fake notes then most of the time they have the same serial and you can then start legal proceedings.
So now you know what to do in this case

No need to pick up soap in return for a sore botty