Riddle #426

Question: A boy at a carnival went to a booth ran by a man who said "If I can write your exact weight on this piece of paper then you have to give me $50, but if I cannot, I will pay you $50." The boy looked around and saw no scale so he agreed, thinking no matter what the carny writes he'll just say he weighs more or less. In the end the boy ended up paying the man $50.

How did the man win the bet?

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