Tom negotiates a deal with the Devil to exchange his soul for a buried treasure chest (although this aspect of the deal goes unstated in the text, but it is explicitly implied and said to be "customary" in these sort of deals). However, the Devil demands certain other conditions to be met as well; specifically, he wants the treasure used "in his service", meaning for sin. He first suggests that Tom buy a slave ship, but when Tom refuses to do so, he instead offers the idea of usury, which is the practice of lending money in exploitative ways, such as by charging unreasonable interest rates.
The text briefly diverges into a local history lesson, describing how the administration of a certain governor was plagued by a lack of physical money, and the ensuing conflict of various business schemes and a credit-based economy led to many people falling on "hard times", which the author puts in quotes in a way of, perhaps, calling out these people for bringing those hard times upon themselves, but then attempting to shift the blame elsewhere.
Tom's customers are specifically listed; the needy, adventurous, those who fail to manage their money properly, and those whose business is based on speculation and investment. Tom's final customer is a "land jobber", an old term for a real estate speculator. The general implication is that these are desperate people.