Background: Some think that when Muslims conquered territory, slaves were often taken or that slavery is allowed in Islam.

When the Quran was revealed, slavery was in wide practice throughout the Arab World, thus, it would have been impractical and detrimental to place an immediate ban on the slave trade, forcing all slaves into unemployment/poverty. The Quran uses a better psychological approach to win people’s hearts in the struggle against slavery and all forms of discrimination by placing a strong emphasis on human dignity and equality and encouraging the freeing of slaves:

Did we not show him the two paths? He should choose the difficult path. Which one is the difficult path? The freeing of slaves. Feeding, during the time of hardship... [90:10-14]

Righteousness is not turning your faces towards the east or the west. Righteous are those who believe in God, the Last Day, the angels, the scripture, and the prophets; and they give the money, cheerfully, to the relatives, the orphans, the needy, the traveller, the beggars, and to free the slaves... [2:177]

The Quran lists the penalty for various misdemeanours as freeing of slaves, see 4:92, 5:89, 58:3. It also implies distribution of community/collective charities would contribute towards freeing of slaves:

The charities are to go to the poor, and the needy, and those who work to collect them, and those whose hearts have been united, and to free the slaves, and those in debt, and in the cause of God, and the traveller. A duty from God, and God is Knowledgeable, Wise. [9:60]

The only method mentioned in The Quran for gaining captives (more correctly termed 'prisoners of war', not slaves) is during warfare, after which they must be released or ransomed. Keeping them is not an option:

Therefore, if you encounter those who deny the truth (in warfare), then bring about the captives until when you have subdued/overcome them, then strengthen the bind. Then after either grace/favour or ransom, until the war lays down its burdens. That, and had God willed, surely He would have gained victory Himself from them, but He tests some of you with others. And those who get killed in the cause of God, He will never let their deeds be put to waste. [47:4]

Interestingly, in the above verse, it implies a preference for taking captives rather than killing the enemy in warfare.

As can clearly be seen, taking all of the above verses of The Quran into account, it would lead to a gradual reduction in slave numbers and eventually abolish slavery.

READ - click to look up verse references