Ibn Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "Whoever frees his share of a slave and has with him the value of the rest of the slave, then the slave is free." Nafi' (may Allah have mercy on him) said: Otherwise, only as much is freed as he has freed.
Ayyub said: I do not know whether this is something Nafi' said or whether it is part of the hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21340