Diggory challenges Damon to a gambling match to win back the money.
This is turning into a production of Guys and Dolls.
So the two men gamble and it's very tense. They even draw a crowd of some locals who think they're both weirdoes for playing dice outside in the dark.
Diggory wins all the money and Damon stomps off, furious to have been bested by a poor chap like Diggory.
Damon stomps off to the highway and sees Eustacia and Clym riding in a carriage towards their new home, laughing happily.
Damon gets even more depressed upon seeing this.
Diggory goes up and talks to Eustacia and Clym while Damon hides.
He tells the Yeobrights that he has a package for Thomasin and Eustacia tells him where she is. So Diggory leaves to give her all the money, since he doesn't realize that some of it is meant for Clym.
"It had not been comprehended by the reddleman that at halfway through the performance the game was continued with the money of another person; and it was an error which afterwards helped to cause more misfortune than treble the loss of money value could have done" (3.8.79).