At first she was excited at the thought of finding the cash.
But then she realised it was not real when she noticed that special security features were missing.
The young detective gave the note to her mother, who handed it into a police station.
A spokesman for the police in Siegburg, western Germany, said: "Apparently it was not very good, but it was printed on both sides which is better than some forgeries you get."
But this time honesty didn't pay - a police spokesman said the girl wouldn't be getting a reward.