How much did an onlooker offer to pay Thornton for Buck? Does Thornton accept?
In Chapter 6, John Thornton has Buck attempt to pull a sled weighing 1000 pounds 100 yards. He is doing this as a bet. Before and after Buck does this, a man offers Thornton money for Buck.
Before Buck even tries to pull the sled, the man offers $800 for him, but Thornton refuses. After the test, after Buck has managed to pull the sled, the man makes two separate offers. First, he says he'll give $1000 for Buck, then he offers $1200. All Thornton tells him is "You can go to hell, sir." In other words, he refuses the offer.
in chapter 6, For the Love of a Man, Buck pulls a sled weighing 1,000 lbs. 100 feet after breaking it out of the ice, and wins John Thornton $1,600 in five minutes, a Skookum Bench King (a King is someone who struck it rich, Skookum Bench is the stream that this specific King struck it rich at) offers John Thornton $1,000 and then $1,200, but Thornton tells him no, "No, sir. You can go to hell, sir. It's the best I can do for you, sir." (he refused the offer). This bet allows Thornton to be able to pay off certain debts and go searching for a legendary lost cabin where the highest quality gold could be found. However, very few people have returned from it.
All of this is in the book
Before Buck started, the Skookum Bench King (who was the latest person to strike it rich) offered John Thornton $800, but Thronton refused.
As an additional note, all of the monetary references were paid in gold dust, flakes, or nuggets