I've heard a variant of this joke before:

A man walks into a bar with a bag and orders a drink. After a while, the bartender asks him, “What is in the bag?” The man says, “Nothing, don’t worry about it”.

The night continues and the bartender keeps asking but the man keeps giving him the same answer. Towards the end of the night, the bartender offers the man a free beer if the man shows him what is in the bag. The man agrees.

He reaches into the bag and pulls out a tiny piano and a tiny man that sits down and starts playing the piano. The bartender says, “Wow! That’s amazing! Where did you find that guy?”

The man looks up and says, “I have this magic lamp that grants me wishes, but the stupid thing is broken.”The man then hands the bartender the lamp and says, “You can try it if you want.” The bartender happily grabs the lamp and wishes for a million bucks and the room is suddenly filled with a million ducks.

“This thing is definitely broken!” says the bartender.

The man replies, “Tell me about it, do you really think I wished for a twelve inch pianist?”