The Kommandant Dies at Dawn - S5-E6
Trivia: This is not really a plot hole but more of a plot device: The episode revolves around using the unsuspecting Klink as a courier by hiding secret information in his belt. This episode however is the only time in the series Klink wears a belt on his coat. In all other episodes, he doesn't wear one.
Answer: It's a comedy, not a documentary.
stiiggy
Perhaps it was counterfeit. There are numerous episodes where they deal in counterfeit monies.