Carnival

 

 
A boy at a carnival walked up to a booth where a man said to the
boy, " If I write your exact weight on this peice of paper
you will have to give me 50$ if I cannot I will give you 50$
dollars."
the boy looked around and seen no scale- so he played thinking no
matter what the carny writes he'll just says he weighs more or
less.
in the end the boy payed the man 50$.
how did the man win the bet?