Apostate prophet battles injustice; why does Allah allow slavery?

In answer to the video "Islamic Slavery"

From a higher perspective, the Quran explains the reason for there being differences socio-economical, or even psycho-physical differences among humans. All human beings were created with the aim of being Allah's vicegerents on earth. We have been endowed with freewill -which asserts itself through tests and trials- and are compelled to use it to fulfill our role of vicegerency according to our degrees in this world

6:165"And He it is Who has made you successors in the land and raised some of you above others by (various) grades that He might try you by what He has given you".

This is according to the divine principles of the Greatest of Judges 11:45,95:8 that what is expected of a person is directly in correlation to that person's condition 65:7.

Just like some trees are made to excell others in fruits 13:4, we are raised in degrees and some of us made to excell others in certain aspects, whether physical or mental, social or economical etc, but it is all done according to an all encompassing wisdom and knowledge that takes into consideration every aspect of our being 6:83. However, if one disregards the materialistic mindframe that blurs the higher realities of existence, the level of difficulty in all cases can be said to be the same; the one with less wordly benefits (financially, in his health etc) has less avenues by which to be religiously and morally accountable but he is required to have a high level of inner discipline, patience and trust in God while the reverse is the case for the more advantaged in terms of wordly benefits; the burden on his inner discipline and steadfastness is less while the means given to him by which to be morally tested are manifold. He can be tested in his wealth, his health and other privileges.

A great part of Allah's scheme of testing the people through their choices in order to gradually purge the hearts, is to test us through our dealings with eachother

25:20"and We have made some of you a trial for others; will you bear patiently? And your Lord is ever Seeing".

One of the obvious proof for this is how most often individuals are endowed with benefits, material, physical or abstract, surpassing the basic and reasonable needs. All else will either vanish or be left behind after their passing away for others to benefit from. Through this scheme every human being becomes by virtue of his social existence, a means whereby the moral qualities of his fellow men are put to a test

"I saw Abu Dharr al-Ghifâri wearing a cloak, and his slave, too, was wearing a cloak. We asked him about that (how both were wearing similar cloaks). He replied: Once I abused a man, and he complained of me to the Prophet. The Prophet asked me, Did you abuse him by slighting his mother? You are a man who has jahiliyah. He added: Your slaves are your brethren upon whom Allah has given you authority. So if one has one’s brethren under one’s control, he should feed them with the like of what he eats and clothe them with the like of what he wears. You should not overburden them with what they cannot bear, and if you do so, help them (in their hard job)".

As the HB puts it in

Prov22:2"The rich and poor meet together: the LORD is the maker of them all".

God is not only the maker of every human, but also of their conditions, through the laws of the material and physical world He has created, in which almost every outcome is the result of a previous action. The people should therefore, regardless of their circumstances keep in view that ultimate reality in their dealings with oneanother instead. These wise words however are clearly bellied by other passages in the HB discriminating among the people solely on the basis of appearance and physical condition Lev21:16-24.

Just as there is diversity in nature, as in the aforementioned metaphor of the fruit trees, mankind is bound by that system too 6:165,11:118,30:22. As said many times in the Book, mankind could have been made a single nation, equal in all aspects, and spiritually upright 42:8. But Allah has decreed there will be diversity in creation whether physical, cultural, material or spiritual through the process of freechoice which was divinely established, sustained every step of the way and encompassed by Allah's power. God is thus ultimately the cause of these differences. 

Here is how that causality plays out. For example;

- Some people have been made by Allah to resist better to diseases, others are stronger, taller or more intelligent. This is due to the natural biological processes He established, that can either be triggered by internal reactions and mutations or/and influenced by external, environemental factors that are themselves subject to the divinely decreed law of causation which God dominates with His all encompassing wisdom and power at each instant.

- Some people have been made by Allah to believe, and others to err and this is due to the system of freewill He established and fully controls, according to which one's moral choices shape his spiritual condition and destiny, either darkening his inner spiritual receptivity or enlightening it and making it further receptive to external guidance.

- Some people have been made by Allah to enjoy more wordly, material benefits than others. This again, is the consequence of a chain of causality in God's grasp.

This diversity however isnt viewed in the Quran as a stain purposefully put on the human race. Neither is it allowed by God in order to confuse and seperate people, or oppose them. Rather it is allowed, as an outcome of the established system of causality, because it is a driving force that creates interraction, interdependency, exchanges and positive understanding

49:13"We have created you of a male and a female and made you nations and tribes that you may know each other. Indeed, the most honourable among you in the sight of Allah is the most fearful (of Allah) among you. Indeed, Allah is Knowing, Aware".

This is one of the most compelling Quranic proof man is repeatedly told to ponder upon; the divine unity in the face of an interdependant diversity

43:32"it is We who distribute their means of livelihood among them in the life of this world, and raise some of them by degrees above others, to the end that they might avail themselves of one another's help".

This is based on the wisdom that no human being should become independent of others, but everyone should remain dependent on the other in various ways. The Creator is in this way in total contrast with His creation; He is the Self-Sufficient, Sustainer of the universe at each instant. He creates difference among His creation, through the natural processes He established, and the system of freewill on which He has full control.