How did the man win the bet?

A boy was at a carnival and went to a booth where a man said to the boy,
“If I 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 looks around and sees no scale so he agrees,
thinking no matter what the man 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?