...that he'll pay him the thousands of dollars he owes him for backgammon? What's the particular episode?

Hurley is playing Walt in backgammon and Hurley walks off, Walt shouts at him saying he owes him 17000 dollars. I think the episode was Raised By Another, Claire's episode.

In the episode "In translation" Walt also tells Locke that Hurley owes him a lot of money

Yes, I think thats right but do you think Walt believed he would get the money or do you think he is like charlie.

no i dont think that Walt thought that he was actually going to get the money but i bet he could expect a nice gift if they were rescued and taken back. it was a classic line though by Walt because when Hurley says yeah youll get it, we dont know hes a millionaire.