Sayyiduna Ibn Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) decided: "Any slave who is jointly owned, and a man frees his share, the value is set as of the day of freeing, not at the time of death."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21389