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.