Until we have some valid documentation, I would suggest we ignore sites merely pushing an anti-Catholic or dissident agenda.
Here's the quote you will often see, and even though an alleged primary source is cited (Instruction of the Holy Office of June, 1866) there is NO link pointing to the actual primary source.
Slavery itself, considered as such in it essential nature, is not at all contrary to the natural and divine law, and there can be several just titles of slavery, and these are referred to by approved theologians and commentators of the sacred canons. It is not contrary to the natural and divine law for a slave to be sold, bought, exchanged or given.Many of those sites which cite/quote this "source" also make a point to say "Signed by Pope Pius IX himself." But did he sign any such thing? Does any such document really exist? Or, is this a lie which has been foisted upon us for so long and no one really questioned the veracity of the claim?
After publishing the above, I did come across a site which has more context - and the context puts a bit of a different light upon the topic on the Kings College site:
1866 Instruction of the Holy Office in Response to questions form the Vicar Apostolic of the Galla tribe in Ethiopia
... slavery itself, considered as such in its essential nature, is not at all contrary to the natural and divine law, and there can be several just titles of slavery and these are referred to by approved theologians and commentators of the sacred canons.So, there's a bit more context, perhaps - but still not the whole document. It is interesting to note that this document is not supporting the sort of slave trade that most Americans think about, for those "traded" in America were unjustly deprived of their freedom.
For the sort of ownership which a slave-owner has over a slave is understood as nothing other than the perpetual right of disposing of the work of a slave for one's own benefit - services which it is right for one human being to provide for another.
From this it follows that it is not contrary to the natural and divine law for a slave to be sold, bought, exchanged or donated, provided that in this sale, purchase, exchange or gift, the due conditions are strictly observed which the approved authors likewise describe and explain.
Among these conditions the most important ones are that the purchaser should carefully examine whether the slave who is put up for sale has been justly or unjustly deprived of his liberty, and that the vendor should do nothing which might endanger the life, virtue or Catholic faith of the slave who is to be transferred to another's possession.