Ibn Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "Whoever frees a share of a slave, it becomes binding on him to free the rest of the slave as well. If he has that much of the price with him, a fair value shall be fixed, the other partners shall be given their shares, and the freed slave's way shall be left free."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21347