Edit: Found. FUKUJUU NO GAKUEN

A complete manga, a few chapters long, a man has sex with a debt collector to be able to help his master keep their house/mansion. His current master is the son of the man who saved his life, or something along those lines, and he is a young student/scientist.

I believe his master sells the house in the end but the man continues to have sex with the debt collector in order to get the house back.

I've already looked at the debt/debt collector tags.

The only other thing I remember is that the art was pretty manly. Not like bara or anything. Just that no one was feminine or cutesy.

Bump?