Sa'id (may Allah have mercy on him) narrates with his chain that the Messenger of Allah (peace and blessings be upon him) said: "Whoever frees his share in a slave, it is binding upon him to free the remainder as well, if he has wealth. If he has no wealth, a fair value is to be set, and the slave is to be made to work for the remaining share, but without being overburdened."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21370