When Buck and Thornton go to Circle City, the locals bet on Buck to:

I think this happens in Dawson. In the Eldorado Saloon, men were bragging about their dogs when a man named Matthewson claimed his dog could start and walk off with a sled loaded with seven hundred pounds.

......."Pooh! pooh!" said John Thornton; "Buck can start a thousand pounds"

Buck ends up winning Thornton $1600.