There are a number of different ironic elements about Tom's willingness to become a userer but not a slave trader. One such irony is that Tom is unwilling to exploit African slaves but is perfectly happy to exploit the poor, often condemning them to a similar fate to slavery. In colonial America almost every colony operated debtors' prisons which sent people who could not pay their debts to prison until they were able to pay. Although Tom Walker was unwilling to participate in the slave trade, he was willing to participate in a very similar system that would have robbed his clients of their freedom and consigned them to forced labor. Washington Irving no doubt chose these two professions to highlight his strong distaste for slavery (which even Tom Walker wouldn't agree to be involved in) but also to highlight the similarities and injustices of the practice of usury in early 19th century America. This serves to remind us of both the true horror of the slave trade but also highlight the ways in which people were also exploited due to their socio-economic class.