Probably a good idea if the idea of slavery in a medieval setting isn/ for you cause further in the story the enemy pillages civillian villages, rapes the women, and captures able bodied men to be slaves while they occupy a fortress. All under the order of the Crown Prince. The story doesn't shy away from the more unsavory parts of medieval settings, esp with how corrupt the enemy kingdom is. This all happens in the 1st volume of the VN btw.
The existence of slaves and the usage of "unsavory parts of medieval settings" isn't the problem. The problem is the dime-a-dozen protagonists who are willing to own another person with complete obedience due to magic.
The problem is the author trying to justify something that they could have otherwise written differently.
But the fantasy setting demands a beastgirl character in its cast, so what's the easiest way to do that? Let's just have the protagonist BUY A SLAVE with no hesitation.
The author could have written anything else to add a character capable of helping them. They could have bought her freedom, and the slave would feel indebted to her and stay in the party. Or they could have purchased her and broken the magic contract. Or they could have broken her free
without monetarily supporting the local slavery business, and fled the town they rescued her from. Hell, they could have written her to not be a slave in the first place!
The author is saying, "See, slavery is okay here because the slaves are healthy and educated, and because they choose to be slaves to pay off their debts." That's one hell of a stance to take. Having your protagonist purchase a slave, and claim that the slaves are happy to be slaves is just lazy writing. It's a problematic trope that's way too prevalent in the genre.