Alqamah bin Abdullah al-Muzani (may Allah have mercy on him) narrates from his father (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "A man may free as much of his slave as he wishes: a third, a quarter or a fifth. There is no compulsion between Allah and that servant."
(b) Abu al-Walid (may Allah have mercy on him) said: Whatever share the deceased has freed, nothing else will cancel it; that much will be free.
This is a good interpretation, except that this Muhammad bin Fada' is da'if (weak) and is not used as proof; Yahya bin Ma'in, Sulayman bin Harb and Abu Abd al-Rahman al-Nasa'i (may Allah have mercy on them) criticised him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21320