Comment by sosocrates on Mushoku Tensei - Isekai Ittara Honki Dasu - Chapter 58: Unspeakable Strength

In their verse slavery is common and they had to buy a child slave since it's one of the requirements to ensure that they achieve their goal

I mean, apart from the fact that slavery(aside from 'criminal slavery' wherein it really depends on conditions) itself is a morally 'bad', I would say that buying her is a neutral/positive thing. Doing so prevents her from dying and/or falling into the hands of pedos.