Salafist Sheikh Wissam al-Masri is mediating the release of 25 police and soldiers held by IS and the Syrian branch of Al-Qaeda, the Al-Nusra Front.
Masri told reporters IS wrote a letter to him demanding the release of Saja al-Dulaimi, the former wife of IS leader Abu Bakr al-Baghdadi, as well as Ola Sharkas, the wife of Al-Nusra commander Anas Sharkas.
The jihadists also called for "the liberation of all female Muslims detained in Lebanon in connection to the war in Syria," Masri said.
Signup to our newsletter and follow us on Facebook and Twitter!
Lebanese authorities announced earlier this month the arrests of Dulaimi and Sharkas's wife. Dulaimi had tried to enter Lebanon from Syria with two sons and a daughter, the interior minister said.
The soldiers and police were captured when Syria-based militants briefly overran the Lebanese border town of Arsal in August.
The fighters withdrew after a truce negotiated by clerics, but took 30 hostages.
Four have since been executed, while a fifth died from his injuries in captivity.