Djinn Master won't feel regret for treating MC like a mindless beast; doesn't actually understand what he did was horrible.
Best to karma strike him where it'll real hurt: making the deal all for nothing. Make it that he not only lost claim of MC but on the land he gave a lot for. Or that MC's new pack find even better lands to expand in some part due to MC, lands that would make the djinn envious. π
(A/N: this is just me making crack theories of what would be fun karmic slapbacks for DM's actions.)
Actually, the Djinn master is quite content with what he got. He wanted the city and he has hardly any interest in the countryside or woods, which are land that werewolves want much more.
I will say he feels some small amount of regret once he comes to see MC as a human being too and not an animal. (MC always stayed in wolf form, so that helped the djinns view them as a beast). He still doesn't view the way he treated MC as abusive, though. Nor does he regret keeping them basically imprisoned for so long.
Just... he does understand, in a way, that MC suffered under his care. And he can understand why MC would feel hate and/or fear towards him.