The story makes clear that when Tom strikes his bargain with the Devil, he uses the money that he gains to become a usurer; that is a man who lends out money at great interest in order to receive even more money in return. This profession is suggested by the Devil after Tom refuses to deal in the slave trade. However, Tom shows no such scruples when it is suggested that he becomes a usurer, and indeed, the Devil is delighted that he has found such a willing acolyte, as demonstrated by Tom's promise to charge disproportionate amounts of interest and to drive everybody he lends to to ruin. The text describes how he would send those who borrow from him away from his house "as dry as a sponge" once he has taken all the money he can from them. However, even though he suddenly finds himself incredibly wealthy, the text makes clear that this does nothing to change his character and the kind of person that he is. Note that he is still just as miserly as he was when he was poor, as the following quote demonstrates:
He built himself, as usual, a vast house, out of ostentation; but left the greater part of it unfinished and unfurnished out of parsimony. He even set up a carriage in the fullness of his vain glory, though he nearly starved the horses which drew it; and as the ungreased wheels groaned and screeched on the axle trees, you would have thought you heard the souls of the poor debtors he was squeezing.
The irony is clear: Tom is now a man who is spectacularly wealthy, but he is so precious about his wealth that he does not even bother to feed his horses properly because he would rather keep that wealth rather than spend it. In spite of the massive change in his circumstances, his original character as a miser has clearly not been altered in any way, and his love of money is still shown to be dangerously extreme. He is easy prey indeed for the Devil, and thus the reader feels he receives his just desserts at the end of the story when the Devil comes to claim him for his own.