Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says: "When a slave gives his wife two divorces, the woman becomes unlawful for him until she marries another man and becomes widowed or divorced, whether the woman is free or a slave-girl. However, the waiting period (‘iddah) for a free woman is three menstrual cycles, while for a slave-girl it is two menstrual cycles."