Muwatta Malik 1208

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ مَنْ أَذِنَ لِعَبْدِهِ أَنْ يَنْكِحَ فَالطَّلاَقُ بِيَدِ الْعَبْدِ لَيْسَ بِيَدِ غَيْرِهِ مِنْ طَلاَقِهِ شَىْءٌ فَأَمَّا أَنْ يَأْخُذَ الرَّجُلُ أَمَةَ غُلاَمِهِ أَوْ أَمَةَ وَلِيدَتِهِ فَلاَ جُنَاحَ عَلَيْهِ ‏.‏

Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, "If a man gives his slave permission to marry, the divorce is in the hand of the slave, and nobody else has any power over his divorce. Nothing is held against a man who takes the slave-girl of his male slave or the slave-girl of his female-slave