Plot in The Devil and Tom Walker
Irving shows that Tom and his wife are not the only people in the area who are greedy. In fact, it seems that many in New England are looking for ways to get rich quick regardless of the moral cost. The resulting economic depression makes people more desperate for money, which works in Tom's favor. He's able to lend money at even more exorbitant rates because people are willing to deal with him even on bad terms.
The Massachusetts Land Bank was a paper-money experiment for poor debtors, particularly farmers, to get out of paying their debts and taxes by creating cheap paper currency based on land instead of gold or silver. This rush for money in a time of economic depression provides the perfect situation for Tom walker to work as a usurer.
Readers might wonder why the devil didn't deal with Tom's wife, especially considering how she was just as greedy, if not more so, than Tom. An explanation might be that the devil wants to ruin as many people as possible, and so letting Tom use Kidd's treasure to become a usurer would make him a socially influential person that could cause a lot of harm. Tom's wife wouldn't have been able to hold such a position due to the social restrictions enforced on women at the time.