Malik said that it was not permissible for a slave to buy himself from his master on the provision that he could give the wala' to whomever he wished as the wala' was for the one who set him free, and that had a man given permission to his mawla to give the wala' to whomever he wished, that would not have been permitted, because the Messenger of Allah, may Allah bless him and grant him peace, had said, "The wala' is for the one who sets free. " The Messenger of Allah, may Allah bless him and grant him peace, forbade selling or giving away the wala'. For if it was permitted to the master to stipulate that for him and to give him permission to give the wala' to whomever he liked, that would be a gift.
USC-MSA web (English) reference: Book 38, Hadith 20