When the Devil insists that Tom use his new-found wealth to serve him, the miserly old man does not argue. The Devil proceeds to suggest that Tom become a slave dealer to fulfill his service requirement, but Tom balks. As a compromise, Tom agrees to become an usurer for the Devil. An usurer is an unscrupulous money-lender who lends money at unusually high interest rates.
Accordingly, Tom becomes a broker in Boston. He becomes known far and wide for being a 'ready moneyed man.' However, Tom is a miserly businessman. This is shown in the way he lends out money: the needier his customers, the harder his lending terms. Lending terms usually include interest rates and special fees: this is money paid on top of what is borrowed, for the privilege of borrowing the money). So, it is implied that Tom's most poverty-stricken customers suffered all the more for borrowing from him.
From this newly accumulated wealth, Tom builds himself a 'vast house, out of ostentation.' To be ostentatious is to showcase a flamboyant and vulgar (tasteless) display of wealth. This ostentatious behavior highlights Tom's pride. Irving also writes that Tom sets up a new carriage 'in the fullness of his vainglory.' This description again highlights Tom's pride in his newly acquired prosperity.
Tom's stinginess is also highlighted in the way he has chosen to leave the greater part of his magnificent house 'unfinished and unfurnished out of parsimony.' Parsimony is just another word for stinginess or miserliness. In other words, Tom wants to display his wealth without the expense of actually spending the necessary amount of money to make it happen.
He also starves the horses responsible for pulling his carriage. Additionally, Tom is so stingy that he foregoes the expense of greasing the wheels of his carriage, with the result that
...as the ungreased wheels groaned and screeched on the axle trees, you would have thought you heard the souls of the poor debtors he was squeezing.
Tom's pride and stinginess is well illustrated in the ways described above.