Carnival Trick

Question: A boy was at a carnival and went to a booth where a man said to him, "If I write your exact weight on this paper, then you give me $50, but if i cannot, I'll pay you $50." The boy agreed, figuring he would just say he weighed more or less than anything the man wrote. In the end, the boy owed the man $50. How did the man win the bet?

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