Salim bin Abdullah narrates from his father (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "If a slave belongs to two men and one of them frees his share, then if he is wealthy, the highest value or a fair value shall be fixed, with neither reduction nor excess. Then he shall be charged with paying the remaining shares."
(b) Imam al-Shafi'i (may Allah have mercy on him) said: The highest value shall be fixed and the slave shall be freed, but there shall be neither reduction nor excess in the value.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21328