Someone actually posted a great informative comment on slavery when I made a thread concerning this. Here it is if you or anyone else is interested. I found it interesting.
“As for slavery, the form that is an intrinsic evil is defined in the CCC as when people are “being bought, sold and exchanged like merchandise, in disregard for their personal dignity. It is a sin against the dignity of persons and their fundamental rights to reduce them by violence to their productive value or to a source of profit.”
While the word “slavery” has come to exclusively carry that narrow meaning, in the past it had broader connotations that included just and reasonable titles to services.
One can acquire title to reasonable services justly (for example, through a voluntary exchange) and buy, sell, and exchange title to services. None of that violates the natural law. However, experience came to show that when such title was life long and the scope of service expansive, title was often not acquired justly and people were treated as merchandise/chattel (rather than their services) and in general not treated as human beings (see e.g. American slavery). It was also often accompanied by other evils. As the article below explains, the “just” slavery was almost exclusively theoretical and so you see general prohibitions on it from the Church after a certain period.
Even after this consensus, it was also sometimes considered a tolerable evil that would result in greater evils and disorder in society if quickly abolished. The wisdom or lack thereof in that position is a question of fact, not revealed truth. In the US, I think we can all agree abolition was well-worth its immediate and consequent high cost to the country (ie the Civil War and its aftermath).
More here:
http://www.newadvent.org/cathen/14039a.htm
The Church still does not condemn the ownership of title to someone’s reasonable services while treating them with human dignity. In fact, it happens everyday just in a more limited scope.“