In "The Devil And Tom Walker", what bargain does Tom make with the stranger in the forest?
Tom wants the treasure, and he will agree to almost everything. The one thing that the stranger, the devil, absolutely insists on is that the money from the treasure be used for service to him (the devil) He proposes to Tom that Tom outfit a slave ship and deal in the slave industry. Tom absolutely refuses.
".....the devil himself could not tempt him to turn slave dealer." ( paragraph 37)
The devil backs off this idea and proposes that Tom should become a lender of money. He would charge high interest to those who could not afford it and foreclose on their property. The devil suggests two cents a month interest, but Tom thinks he can get four cents a month. The devil tells him,
"You shall extort bonds, foreclose mortgages, drive the merchant to bankruptcy." (paragraph 44)
For this he would receive the devil's treasure, but only under certain conditions. Washington Irving never tells the reader what those conditions are, but he does say that they are serious enough that Tom Walker had to think about them.
"What these conditions were, may easily be surmised, though Tom never disclosed them publicly. They must have been very hard, for he required time to think of them." (paragraph 22)
However, it is strongly foreshadowed (hinted) that he sold his soul to the devil, and his place for eternity would be someplace dark, sooty, and uncomfortable.