Allah Almighty says: {Yet he did not make his way through the steep path, and how do you know what the steep path is? It is freeing a slave.} [Surat al-Balad: 11-13]
1) We are encouraged to free slaves in pursuit of Allah’s pleasure. This is one of the difficult acts of worship, as a person would give away part of his dear possessions.
2) He who gives charity willingly is one of those who break through the difficult pass and purify themselves. Allah Almighty says: {He who purified it has indeed succeeded.}
In this regard, there are two Arabic terms: 'fakk raqabah' and '‘itq'. The first term refers to the case when a person shares with another person the freeing of a slave; whereas the second term refers to the case when a person frees a slave alone. In a Hadīth, the Prophet (may Allah’s peace and blessings be upon him) explained this distinction, saying: “Do ‘itq or fakk raqabah.” It was asked: “O Messenger of Allah, are the two not the same?” He said: “No, in ‘itq, you free a slave alone; whereas in fakk raqabah, you contribute to freeing a slave.” [Narrated by Ahmad] Ibn Kathīr cited it in his commentary on the above verse.
1358/1 - Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah’s peace and blessings be upon him) said: “He who sets free a Muslim slave, Allah will deliver from Hellfire every part of his body in return for every part of the slave’s body, even his private parts.” [Narrated by Al-Bukhāri and Muslim]
1) Islam encourages its followers to set slaves free, which refutes the claims of those who criticize Islam and accuse it of injustice on account of slavery.
2) Recompense is of the same type of action, and the bounty of Allah is greater still.
3) We are exhorted to emancipate a slave free from defects, so that we can get the reward in full. This is the attitude of the believers who are keen to seek the full bounty of Allah by every means possible.
1359/2 - Abu Dharr (may Allah be pleased with him) reported: I said: “O Messenger of Allah, which deed is the best?” He said: “Belief in Allah and Jihad in the cause of Allah.” I said: “Which slave is the best?” He said: “The most precious in the sight of his master and the most expensive.” [Narrated by Al-Bukhāri and Muslim]
1) Freeing a valuable and expensive slave is one of the most virtuous deeds in the sight of Allah Almighty, as only a person with true faith would do this. He gives what he holds as most dear in order to win the approval of his Lord.
2) One of the means whereby a person can purify his soul and get rid of stinginess and avarice is to give away what he holds dear.
Here I present a comprehensive summary on the slavery system in Islam, taken from the words of the erudite scholar Al-Tāhir ibn ‘Āshūr (died 1393 A.H.) (may Allah have mercy upon him) in his book "The Foundations of the Social System in Islam": “When Islam came, slavery had been deeply-rooted in the structure of nations and part of the history of their civilization. So, if it had abolished slavery all at once, this would have caused great turbulence among Muslims and the surrounding communities that were linked to Muslims. That is why it adopted a gradual approach suitable for the Fitrah. It abolished the optional and compulsory causes of slavery, leaving only one cause, namely taking captives in wars between Muslims and disbelievers. On the other hand, Islam increased the causes of emancipation. Based on the Shariah rulings and teachings in regards to the conditions of slaves and their manumission, the scholars deduced this rule: The Legislator favors freedom. Before Islam, there was no law, religious or man-made, to lay down rights for slaves or protect them from harms as the Muslim Shariah has done.”
He also said in his book "The Objectives": “So, Islam worked to reconcile its two objectives of spreading freedom and preserving the world order. It made the causes of freedom dominant over the causes of slavery, thus resisting and reducing slavery and improving whatever remains of it.” Then, he tackled the issue in some more details, which I recommended to read.