First there's the stupid idea of pushing debt to a kid like that, but they're thugs, so it's not that out of the question.
Then the thug turns out to be completely reasonable, just having a slightly black job.
Then lawyer shows up and says it's illegal, which is surprisingly reasonable.
Then hippity, hoppity, you're now my property. Wait, what? If the debt is illegal in the first place, then solving it can't make him owe anyone anything except gratitude that's paid in full by saying, "Thank you."
When a manga goes out of its way to say how things don't work one way, and then turn around to make the plot hinge on it. It's like the Quintuplets author all over again.
At least the girl wasn't as stoic as she first appeared.