Good concept
Yes - I think the concept is sound. It actually gives you a lot of options in terms of story (and backstory).
For example, as the story unravels perhaps you could show that it is the Debt Collector who has gradually, over the years, destroyed the life of the debtor and now this is "payback" (ha!) time. Maybe the debtor has been forced to give up one treasured possession after another until he (she?) has lost everything. (That's another option - to mix genders).
A slow-reveal that turns the story on its head from one baddie to the other will keep the story out of the typical revenge-mill and probably help in the writing...
Other options could be that it is the Debt Agency from Hell (literally) after the Debtor made some (forgotten?) pact with the Devil...
If you want to keep the Debt Collector as the nice guy, perhaps the Debtor was a bully to the nice-guy in their school days and wants to keep the upper hand.
Try having a look at CAPE FEAR (either version) for an example of a revenge story with menace... It might help inspire.
All the best with it.