Al-Hasan (may Allah have mercy on him) says regarding a jointly owned slave: No one should make a kitabah contract with the slave without his partners. If anyone does so, it will be divided among them all, and the slave remains jointly owned.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21692
Al-Hasan (may Allah have mercy on him) says regarding a slave jointly owned by three men, one of whom made a kitabah contract with him: what he took is taken from him and divided among all (the partners), and the slave remains jointly owned; his kitabah is not valid.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21693
Al-Hasan (may Allah have mercy on him) says regarding a slave jointly owned by two men: Neither of them should make a kitabah contract without his partner's permission. If he does, it will be divided between the two of them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21694