Maliks Muwatta Book 38, Hadith Number 12.
Malik related to me that he had heard that Abdullah ibn Umar was asked whether a slave could be bought on the specific condition that it was to be used to fulfil the obligation of freeing a slave, and he said, “No.”
Malik said, “That is the best of what I have heard on the obligation of freeing slaves. Someone who has to set a slave free because of an obligation on him, may not buy one on the condition that he sets it free because if he does that, whatever he buys is not completely a slave because he has reduced its price by the condition he has made of setting it free.”
Malik added, “There is no harm, however, in someone buying a person expressly to set him free.”
Malik said, “The best of what I have heard on the obligation of freeing slaves is that it is not permitted to free a christian or a jew to fulfil it, and one does not free a mukatab or a mudabbar or an umm walad or a slave to be freed after a certain number of 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 47 ayat 4) The favour is setting free.”
Malik said, “As for obligations of freeing slaves which Allah has mentioned in the Book, one only frees a mumin slave for them.”
Malik said, “It is like that in feeding poor people for kaffara. One must only feed muslims and one does not feed anyone outside of the deen of Islam.”