Salim narrates from his father (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "Whoever frees a slave he shares with another, a fair value shall be fixed from his wealth, with neither reduction nor excess. If he is wealthy, the slave's freedom is his responsibility."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21330