When Tom Walker meets the Devil, the Devil tells him, "I am the great patron and prompter of slave-dealers." Therefore, Irving suggests that slave traders are diabolical and are associated with the Devil himself. Later, Tom Walker is haggling with the Devil about what to do with the buried treasure the Devil will give him. At first, the Devil suggests that Tom Walker should use the money to become a slave trader:
He proposed, therefore, that Tom should employ it in the black traffic; that is to say, that he should fit out a slave-ship. This, however, Tom resolutely refused; he was bad enough in all conscience, but the devil himself could not tempt him to turn slave-trader.
Even Tom Walker, who is totally amoral, cringes at the thought of fitting out a slave ship and becoming a slave trader. Tom eventually becomes a heartless money lender, but this profession, evil though it might be, is implied to be better than that of a slave trader. The devil's suggestion that Tom enter the slave trade implies that Irving felt slaving was the most barbaric and evil business that a person could be involved in. If Tom Walker, who is without a conscience, refuses to enter slave-traading, it is understood to be the quintessence of evil and immorality.
Washington Irving makes his disapproval of slavery and the slave trade very clear by making it one of the primary interests of the devil. For example, in the story, the devil describes his activities by saying:
Since the red men have been exterminated by you white savages, I amuse myself by presiding at the persecutions of quakers and anabaptists; I am the great patron and prompter of slave dealers, and the grand master of the Salem witches.
Here slavery appears alongside religious persecution and witchcraft, which indicates Irving's strong distaste for the practice. Further in the story, when Tom wants to sell his soul to the devil in exchange for treasure, the devil suggests that he use the money to buy and operate a slave ship. However, even the rapacious and morally depraved Tom Walker refuses to become a slave dealer. As noted by the narrator, "This, however, Tom resolutely refused; he was bad enough in all conscience; but the devil himself could not tempt him to turn slave dealer." Even Tom Walker, who is described throughout the story as being of low moral character and sells his soul to the devil, refuses to become a slave dealer. Based on Irving's association of slavery with the devil and Tom's steadfast refusal to take part in it, it is clear that Irving wishes to convey to the reader that slavery is an abominable practice.