Praise be to Allah.
Teaching man-made laws or studying them in order to explain their falseness and to distinguish what is right from what is wrong, and to demonstrate the superiority of Islamic sharee’ah and to show that it is complete and encompasses everything that is in people’s interests with regard to their worship and their interactions with one another is permissible, and may be obligatory if circumstances call for it, in order to demonstrate what is true and what is false and to explain that to the ummah and make them aware of that so that they will adhere to their religion, and will not be deceived by the specious arguments of the deviants and those who promote the rule of man-made laws. It is permissible to take payment for this kind of work.
But with regard to teaching man-made laws because one likes them and wants to promote them, and prefers them to Islamic laws, and opposes Islamic laws, this is a challenge against Allaah and His Messenger, and is blatant kufr and a deviation from the straight path. Taking payment for this is haraam and is evil upon evil. We ask Allaah to keep us safe and sound and we seek refuge in Him from misguidance.
And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions.
From Fataawa al-Lajnah al-Daa’imah, 23/497.
See also question no. 12874.