what is ironic about the arrival of the “black man” at the end of the story?

At the end of the story, a black man on a black horse knocks on his door, come to take Tom away. Tom Walker is gone for good, and when trustees go to claim his assets, they find that all his possessions—including his house—have gone up in flames. The irony is that all of Tom's wealth is meaningless. Tom must commit to his Faustian bargain which demands the Devil always collects his due.