It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (ﷺ) said: "When two people share ownership of a slave and one of them frees his share, if the one who freed is wealthy, then the highest value of the slave will be estimated, and he (the slave) will be freed on behalf of the first (owner)." Amr bin Dinar (may Allah have mercy on him) says: In estimating the value, there should be neither decrease nor increase.
Hadith Referenceالسنن المأثورة / باب من أعتق شركا له في عبد / 550
Hadith Takhrijصحیح بخاری، العتق، باب اذا اعتق عبدا بين اثنين او امة بين الشركاء، رقم: 2521؛ صحیح مسلم، الأيمان والنذور، باب من اعتق شركاً له فى عبد، رقم: 501۔