Sorry, but this makes little sense! In this type of group, even IRL, everyone and specifically the leader NEEDS to know everything about everyone's abilities! But I guess, for shock value we need this "wow, he was amazing and we didn't appreciate him enough"
Only the art is making me come back
Objectively and rationally, I would agree.
But we're talking about a bad leader who once bought a healer slave when he couldn't afford better.
He got all the mileage he could from his "bargain healer", assuming that "bargain" is the same as "defective". (And the fact that MC could only use his heal spell once before collapsing would tend to confirm his prejudice.)
Only when he got rid of his cheap healer and started interacting with other healers did he (very slowly) realize that he had originally bought a diamond in the rough.
So far, it's not that bad. Humans are not fully rational.
The problem is that he had to go the extra mile of dumping him out of the party in the worst possible location and situation.
That's just evil for no reason, and it now comes back to bite him as there is simply no way to get the #1 healer back.
Or so he thinks for now.
Problem is that he just might be able to. The MC still bears the slave mark, which he can only hide.
If they meet, there is a chance that MC won't even have the right to reject an "offer" to get in the party again.
Now that they know how exceptional he is... and now that he doesn't even have the downside of lacking mana... they would likely jump at the opportunity to get him back at no cost.