Nafi' (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with him) that whoever permits his slave to marry, the authority of divorce belongs to the slave alone; no one else holds any authority of divorce on his behalf.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15114