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."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / Q21430