But he still supports the whole slave economy, slave trade, and slavery as an institution by buying a slave. The slave merchant wouldn't care about what a particular customer will do with the slave later. He might start caring if the MC bought all of his slaves and then set them free, as that would include criminal slaves (which the MC himself was, even if unjustified), because those freed slaves going around perfoming new crimes would make people annoyed by the slave trader "allowing" it, even though he didn't free them, he just sold them. Naturally a slave trader wouldn't like a social movement demanding freedom for all slaves, but a dude buying a slave and freeing that single slave doesn't mean anything in the big picture.
In any case this whole slave deal could be the most scatterbrained plot development so far. One of the most unhinged isekai authors I've seen.