I've never read where God considered slavery a sin. I believe God expected those who had slaves to treat them well and allow them the Sabbath to rest.
I would be interested in the scripture that speaks directly to slavery as sin.
The "slavery" meme has been allowed to completely be co-opted and misrepresent or mangled by the Left.
What was a slave in many former societies was, in many cases, nothing more than a servant, such as indentured servants of all nationalities and races which came to this country. Some slaves were permanent, others were temporary. Some offered themselves into slavery in order to survive a harsh political or economic environment, even to ADVANCE themselves, others were conscripted involuntarily.
That slavery wasn't the universal drudgery and misery that history revisionist would like us to believe is made clear by the fact that many ancient societies allowed for freed slaves to bind themselves for life to their master in an act of affection. That many probably did this is testament to the social usefulness of the practice.