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