Slaves Being Second Class
Slavery did not start with Islam. It was already there when Islam came. Islam stopped making new slaves from the prisoners of war and introduced a law of Mukatabat which allow slaves to buy their freedom. Islam did not announced their immediate freedom, because it would disturb the society which depended on slaves for their economy and household jobs. For example, our modern employment is somewhat a form of partial slavery, if a govt declares that no one will employ anyone, there will be a huge unemployment. Same goes for slavery of that time. The the key to get freedom was in the hands of slaves via Mukatabat. Slaves were not free to go any where without the consent of their masters the way an employee has to take leave before going somewhere. Slaves had no right of property. They could be bought and sold and given as gift to others. Their status were raised by Islam and many slaves become authority in Islamic knowledge.