Spice and Wolf.

Just because the "hero" of a story says something does not mean it's a message to the audience. He was trying to be authentic to the time period. Saying you were anti-slavery in that time period would be the same as saying you're "anti-prison" today. It was viewed as a necessary evil.

The author of Spice & wolf was not advocating for slavery in that scene. He was trying to be accurate to the setting the story is based on. In middle ages Europe, slavery was fact of life and a pillar of the economy for centuries. Lawrence himself was almost sold into slavery over a debt.