Why did billy give $2 to Rubin

One day at Grandpa's store, Billy runs into Rubin and Rainie Pritchard, two mean, violent brothers. Rubin is older and quieter, while Rainie is Billy's age, violent and nervous. They start up trouble with Billy and Grandpa, then propose a bet. There is a coon in their part of the country nicknamed the "ghost coon" for seemingly disappearing once he is treed; their blue tick hound has treed him many times only for him to vanish. They bet Billy his hounds cannot tree the ghost coon. Grandpa spots Billy the two dollars and warns the Pritchard boys not to mess with Billy.