As much as I don't like him taking hostages, I can see the possible issues of summoning strangers who might have the power to destroy countries without having a way to control them. I'm surprised our MC didn't ask what happens if the slave owner dies, although I doubt he would get an honest answer if it was beneficial to the MC.
I also don't like that you are kidnapping people to have them deal with your problems, but the story needs to start somewhere.