Imam al-Shafi'i (may Allah have mercy on him) said: "His (the freer's) right of wala' over him has been established, so he has no right to reject his wala' and make him a slave (again); nor may he give it (the wala') as a gift or sell it."
Ibn Umar (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) forbade "that wala' be sold or given as a gift."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21430