Irving writes that Tom never disclosed the conditions of the deal publicly, but notes that the terms of the bargain must have been difficult given the amount of time Tom waited before accepting the deal. After Tom's wife dies, he returns to the swamp to haggle with the devil. The one condition that Irving mentions as being " generally understood in all cases where the devil grants favors" refers to the fact that Tom Walker agrees to give the devil his soul. In return for giving his soul to the devil, Tom is rewarded with Kidd the pirate's buried treasure. However, the devil insists that the money should be used in his service. Initially, the devil proposes that Tom should use the money to fit a slave ship, but Tom refuses. Tom finally agrees to become a heartless usurer and lend money at extremely high rates. Eventually, Tom Walker becomes wealthy by taking advantage of people until the devil finally comes to take him to hell. 

Tom makes a bargain with the devil to get a pirate’s treasure in exchange for “certain conditions.”

Tom Walker does not find it strange when he meets the devil during a walk in the woods.  He does not find anything frightening, even if local legends tell him the area is haunted.  So it should not surprise us that the “black man” who takes him by surprise does not seem to bother him.

The black man told him of great sums of money buried by Kidd the pirate … These he offered to place within Tom Walker's reach, having conceived an especial kindness for him; but they were to be had only on certain conditions.

This proverbial deal with the devil involves conditions which Tom keeps to himself.  They seem to satisfy him, though, because he agrees to them. 

Tom becomes a money lender and is very rich.  One day an ill-conceived oath results in the devil coming to take him, and all of his riches turn to dust.  Irving notes, “Let all gripping money-brokers lay this story to heart.”  Tom’s story should be a warning to anyone who is greedy and takes advantage of others.  If you make a deal with the devil, you are going to get burned.

What is the "bargain" in "The Devil and Tom Walker"?

The "bargain" is what many call a Faustian pact. This takes its name from several version of the story of Faust, a man who sold his soul to the devil in exchange for riches in this life. The story line is similar to Christopher Marlow's "Dr. Faustus" to Goethe's "Faust". It can also be seen in many other stories, from Stephen Vincent Benet's "The Devil and Daniel Webster" to the musical "Damned Yankees" and even to Darth Vader's decision to embrace the "dark side." In "The Devil and Tom Walker", Walker refuses the devil's first "bargain" because he doesn't want to share his money with his wife. However, after his wife is killed, Tom makes a deal with the devil and earns great wealth. However, as in most Faustian stories, Tom eventually regrets his "bargain" and tries to get out of it, but to no avail.