I have wondered this myself. The only thing I can think of is this. Sydney asks Billy if he has any money. Billy says that he has 2 grand.
Each game against the King and the Duck is $2,500. They play and win. They are up $2,500. They play again double of nothing. They are now up $5,000. Perhaps they play again double of nothing. Billy dunks it, they win 10,000. Billy now has $5,000 plus$1,000 of his own money. Maybe Sydney floats him a grand. Billy pays off the bad guys.