Malik related to me that he had heard that Abdullah ibn Umar wasasked whether a slave could be bought on the specific condition thatit was to be used to fulfil the obligation of freeing a slave, and hesaid, "No."Malik said, "That is the best of what I haveheard on the obligation of freeing slaves. Someone who has to set aslave free because of an obligation on him, may not buy one on thecondition that he sets it free because if he does that, whatever hebuys is not completely a slave because he has reduced its price by thecondition he has made of setting it free."Malik added,"There is no harm, however, in someone buying a person expressly toset him free."Malik said, "The best of what I have heard onthe obligation of freeing slaves is that it is not permitted to free achristian or a jew to fulfil it, and one does not free a mukatab or amudabbar or an umm walad or a slave to be freed after a certain numberof years, or a blind person. There is no harm in freeing a christian,jew, or magian voluntarily, because Allah, the Blessed, the Exalted,said in His Book, 'either as a favour then or by ransom,' (Sura 47ayat 4) The favour is setting free."Malik said, "As forobligations of freeing slaves which Allah has mentioned in the Book,one only frees a mumin slave for them."Malik said, "It islike that in feeding poor people for kaffara. One must only feedmuslims and one does not feed anyone outside of the deen of Islam."
USC-MSA web (English) reference: Book 38, Hadith 12
Arabic reference: Book 38, Hadith 1477