They set their terms as follows: When the Devil and Tom Walker first meet in "The Devil and Tom Walker" by Washington Irving, Tom is hesitant to make a deal with him because he is unsure if the man he meets is really the Devil and whether the man is capable of doing everything he claims. Tom's wife becomes very upset when she learns of Tom's hesitation to make the deal, and sets out to find the Devil and make a deal herself. She does not successfully make a deal with the Devil, though, and ends up disappearing. After his wife's disappearance, Tom decides to make a deal with Devil.
Although the Devil does not initially seem very interested in making a deal with Tom, they eventually reach an agreement. Irving never explicitly says Tom sells his soul to the Devil as part of their agreement, instead saying it "was one condition which need not be mentioned, being generally understood in all cases where the devil grants favors."
The Devil insists on a couple other terms in his deal with Tom, too. He wants the work Tom does to make his money to be his work, meaning Tom will be literally doing the Devil's work. At first, the Devil tries to get Tom to agree to be a slave dealer, but Tom refuses. He then suggests that Tom be a usurer, which Tom excitedly agrees to do. They set the following terms:
"You shall lend money at two per cent a month."
"Egad, I'll charge four!" replied Tom Walker.
"You shall extort bonds, foreclose mortgages, drive the merchant to bankruptcy— "
"I'll drive him to the devil," cried Tom Walker, eagerly.