The main cause of slavery originates from the legal war with the disbelievers
23-6-2014 | IslamWeb
Question:
Assalammualaikum I've read the fatwa of slavery in islamweb and i've found that slavery nowdays can only be valid by war between the kuffar. I've never found any other fatwa from this. Is this matter consensus or ijma? And can i have the reference which scholar stated that it is ijma and in which book?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and messenger.
First of all, you should know that a human being is free in principle. It is not permissible to remove such a quality [freedom] from the person except for an Islamic valid reason.
Ibn Qudaamah 
said in Al-Mughni: “According to principle, all human beings are free; Allaah created Aadam and his descendants free, and slavery is an incidental case, and if that incidental case is not known, then the principle ruling is to be applied (i.e. man is free).” [End of quote]
Moreover, the author of Fat-h Al-Qadeer (Ash-Shawkaani) said: “Freedom is the right granted by Allaah, the Almighty, and no one can annul it except by a law of the Sharee'ah, so it is not permissible to annul this right. Therefore, it is not permissible to enslave a free person even if he agrees to this.”
We have clarified the ruling on slavery and its lawful causes in Fataawa 84431 and 86042.
All of the causes that result in slavery originate from the main cause, which is the captivation of non-Muslims during war.
Shaykh Ash-Shinqeeti said in Adhwaa’ Al-Bayaan: “The cause [of slavery] is known - the Muslims imprisoning the non-Muslims in Jihaad.”
In another instance he said: “The cause of the possession of slaves is disbelief and the fighting against Allaah and His Prophet. So, if Allaah enables the Muslims who fight in the cause of Allaah, who sacrifice their lives, property and strength, and all that Allaah has given to them so that the word of Allaah will be the highest over the unbelievers, then Allaah has made them the property of them by way of captivity. This is so unless the Muslim ruler chooses to free them without any compensation or in return for compensation, according to what is to the interest of the Muslims.” [End of quote]
Nonetheless, we have not come across any scholar who has reported a consensus on this matter, but at the same time, we have not come across any scholar who has mentioned a difference of opinion on it.
Allaah Knows best.