Does he who resorts to man-made courts out of personal desire and without deeming it permissible commit an act of disbelief (kufr), or an act of grave sin (fisq)?

Ruling and seeking judgment by other than what Allah has revealed are two sides of the same coin, and neither is permissible. The ruling on them, regarding whether it leads to exiting the fold of Islam or not, varies according to the specific situation. Seeking judgment must be according to the Book of Allah and the Sunnah of His Messenger. So, whoever deems it permissible to seek judgment by other than them is a disbeliever. However, whoever does not deem it permissible but seeks judgment by other than them out of desire for wealth, prestige, or position, is committing a sin. It is not permissible to seek judgment from man-made courts except out of necessity, when no Sharia courts are available. As for the principle, "Whoever does not declare a disbeliever to be a disbeliever is himself a disbeliever," its application is limited to disbelief that is known by necessity to be part of the religion. It does not apply to a Muslim who commits an act of disbelief due to ignorance or a misconception, without the establishment of proof against them.