Sayyiduna Abdullah bin 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 in a slave and also has the wealth, a fair value shall be fixed; otherwise, only as much is freed as he has freed."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21362