Hehe, that is funny. The text on the "donkey note" says:
A donkey that shits ducats
We do not have in Paderborn
But there are enough donkeys in the world
Who buy our paper money.
The "ducat donkey" refers to a famous fairy tale (a wonder donkey that, errm, emits gold ducats). But as in English, Esel/donkey can also mean a stupid person. Ironic and neat.
Christian