What is the Wager that Buck could break out and pull a one thousand pounds load?

The wager is a one thousand dollars,

"And break it out, and walk off with it for a hundred yards," John Thornton said cooly.   "Well," Matthewson said, slowly and deliberately, so that all could hear, "I've got a thousand dollars that says he can't. And there it is." So saying, he slammed a sack of gold dust of the size of a bologna sausage down upon the bar.