Slavery has been a subject that does not have a clear cut view point in the Islamic law.
Step-by-step explanation:
Slavery was a definite part of the Byzantine and mongol life in the area where Islamic law was codified at first so it was seen as something that was necessary.
Most Islamic rulers had their own large body of slaves too, however these slaves were not in the strictest sense of the word unfreeable as they oftentimes became very powerful in their own rights and even started empires after their lords had died.
This meant that the consensus on slaves in Islam has never been reached by the religious law.