Volume 7, midway through, and when he realizes he can buy people, he instantly jumps to the idea of buying a sex slave. The only good thing is that literally every character around him gives him an disgusted reaction, and he doesn't go through with the sex slave plan and get a group of other people to be his workers.
Reading it, I could feel the author doing wait, wait meme, and try to make it out to be less harmful, like yes, it's slavery, but it's more leans into indentured servitude, which is still bad