Slavery does not exist today and a future Caliphate would not attempt to re-introduce it as some misguided people have claimed.
Prophet Muhammad ﷺ came to free human beings from slavery to other human beings, so they become slaves of Allah alone. The Prophet ﷺ said: “Whichever man frees a Muslim man, Allah ta’ala will liberate for each of his organ an organ from the Fire” (Agreed Upon).
Islam’s position on slavery is detailed in the book Shakhsiya Islamiyya volume 2 by Sheikh Taqiudeen an-Nabhani. A brief summary follows.
1- Islam was revealed upon people who owned slaves so the sharia addressed the problem of slavery by giving slaves new rights and restricted enslaving to the sabaya* alone. Previously someone could be enslaved by falling in to debt or as a punishment for a crime or as a prisoner of war, and Islam abolished all of this.
2- Islam encouraged the freeing of slaves and gave it an immense reward.
3- The Islamic sharia legislated “Freeing slaves” as the punishment for a number of crimes and the expiation of sins. For example:
a. Punishing severely such as whipping. The Prophet Muhammad ﷺ said:“Whoever slaps his slave or strikes him, his atonement (kaffara) is to free him”.
b. Breaking oaths
c. Killing a believer accidently
d. Saying to your wife ‘you are like my mother’ then he returns to her
e. Whoever invalidates the fast of Ramadan by sexual intercourse, his atonement is freeing a slave.
4- Islam also legislated a specific category of Zakat to free slaves. The Islamic State which manages the Zakat revenues would spend money from the State Treasury (Bait ul-Mal) to free slaves.
إِنَّمَا الصَّدَقَاتُ لِلْفُقَرَاءِ وَالْمَسَاكِينِ وَالْعَامِلِينَ عَلَيْهَا وَالْمُؤَلَّفَةِ قُلُوبُهُمْ وَفِي الرِّقَابِ وَالْغَارِمِينَ وَفِي سَبِيلِ اللَّهِ وَابْنِ السَّبِيلِ ۖ فَرِيضَةً مِنَ اللَّهِ ۗ وَاللَّهُ عَلِيمٌ حَكِيمٌ
“Zakat is for: the poor, the destitute, those who collect it, reconciling people’s hearts, freeing slaves (fi riqabi), those in debt, spending in the Way of Allah, and travellers. It is a legal obligation from Allah. Allah is All-Knowing, All-Wise.”
Sheikh Taqiudeen an-Nabhani concludes with, “There does not remain even one situation in which enslaving occurs at all (today). Accordingly, Islam has prevented slavery.”
*Sabaya are the women and children who in ancient times accompanied the army in war. It does not apply to female soldiers or prisoners of war. In modern warfare sabaya do not exist so the means of slavery does not exist today.