What seems to be Tom's prime motivation in agreeing to the devil's terms?

Tom Walker's prime motivation for agreeing to the devil's terms is to get his hands on the buried treasure of the pirate Kidd. Tom schemes to keep the treasure for himself and not share it with his wife; this is a function of the story's satirical subplot about marriage. His wife tries to work her own deal with the devil to cut Tom out of the treasure and is killed in her attempted transaction with "Old Scratch."
When he receives the treasure, Tom purchases a grand home that he neglects to furnish, and a stable in which he keeps horses that he is too cheap to feed. 
In return for Kidd's treasure, the devil requires Tom to become a usurer, a person who lends money at very high interest rate, AND the devil will claim Tom's soul when he dies.