وَإِنْ كَانَ مُفَرَّغًا ... كَانَ إِعْرَابُهُ بِحَسَبِ الْعَوَامِلِ.
Translation: If the exception is mufarragh, its case follows the governing elements. (Middle wording is abbreviated.)
A mufarragh construction contains illā after negation or interrogation but omits the named exception base.
Illā does not impose one fixed case on what follows. The actual governor—a verb or preposition—determines the case.
Examples: «مَا جَاءَنِي إِلَّا زَيْدٌ», «مَا رَأَيْتُ إِلَّا زَيْدًا», «مَا مَرَرْتُ إِلَّا بِزَيْدٍ».