It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) said: "If a slave is jointly owned by two men, and one of them frees his share, then the value of the slave will be assessed at a fair price—not excessively high or low—if the other is wealthy. (The wealthy co-owner will be given his share), and thus the slave will become completely free."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4589