How does the bar bet about Buck come to be?

The bet comes about as men are standing about bragging about their dogs and the amount they can haul. John Thornton responds with his own bit of bragging about Buck.... after which, Matthewson calls his Bluff and puts money on it. Thornton doesn't have the money to put up, and he isn't even sure if Buck can do it, but suddenly, the bets on.