Abdullah bin Umar (may Allah be pleased with them both) says: I heard the Messenger of Allah (peace and blessings be upon him); he (peace and blessings be upon him) said: "If a slave belongs to several men and one of them frees his share, the value of the remaining shares shall be assessed from his wealth. If his wealth covers that value, the slave shall be set free."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21323