No, it isn't wrong.
A well-treated slave is a slave nonetheless. I can't see how giving someone opportunities and not freedom is any more benevolent.
Brooks Simpson, a highly respected Civil War historian in Arizona, wrote the other day on his blog about the questions surrounding slavery. Brooks has an ongoing issue with a rather obtuse person and is not one to suffer any fools gladly, so he can be somewhat acerbic. Yet that doesn't mean he's wrong.
https://cwcrossroads.wordpress.com/2013/...n-slavery/
Changing minds on this is futile, so I'm through on this topic.
Best
Rob