This is sanctioned in Islamic law. Now the kidnapped man will be killed, enslaved, exchanged for ransom, or released outright. Here is a salient passage on this issue from a Shafi’i manual of Islamic law:

When an adult male is taken captive, the caliph considers the interests … (of Islam and the Muslims) and decides between the prisoner’s death, slavery, release without paying anything, or ransoming himself in exchange for money or for a Muslim captive held by the enemy. (Reliance of the Traveller o9.14)

A revered Islamic jurist, Al-Mawardi, agrees with Reliance of the Traveller:

As for the captives, the amir has the choice of taking the most beneficial action of four possibilities: the first, to put them to death by cutting their necks; the second, to enslave them and apply the laws of slavery regarding their sale or manumission; the third, to ransom them in exchange for goods or prisoners; and fourth, to show favor to them and pardon them. (Al-Ahkam As-Sultaniyyah (The Laws of Islamic Governance), 4.5)

“Militants kidnap Christian man in Egypt’s Sinai Peninsula,” by Hamza Hendawi, Associated Press, January 17, 2019 9:59 am