Hanafi
The sources give no view for it here.
الجعالة مع جهالة الجعل
هل تصح الجعالة إذا كان الجعل مجهولاً؟
Each school’s answer
Hanafi
Not stated in the sources
Maliki
Not stated in the sources
Shafi'i
Not stated in the sources
Hanbali
Not stated in the sources
The texts
الْجَهْل فِي الْجَعَالَةِ: ٣٦ - الْجَعَالَةُ جَائِزَةٌ عِنْدَ جُمْهُورِ الْفُقَهَاءِ مَعَ مَا فِيهَا مِنْ جَهَالَةِ الْعَمَل لِلْحَاجَةِ إِلَيْهَا؛ لِقَوْلِهِ تَعَالَى: {وَلِمَنْ جَاءَ بِهِ حِمْل بَعِيرٍ وَأَنَا بِهِ زَعِيمٌ} (٢) . هَذَا إِذَا كَانَ الْجُعْل مَعْلُومًا. أَمَّا إِذَا كَانَ الْجُعْل مَجْهُولاً فَإِنَّ الْجَعَالَةَ لاَ تَصِحُّ، عَلَى خِلاَفٍ وَتَفْصِيلٍ يُنْظَرُ فِي مُصْطَلَحِ: (جَعَالَةٌ) .
«الموسوعة الفقهية الكويتية» (Kuwait) entry «جهالة» vol. 16 p. 177
From the schools’ own books
Maliki
The sources give no view for it here.
Shafi'i
The sources give no view for it here.
Hanbali
The sources give no view for it here.
What bears this mark is in our words, our standard wording; the books’ texts stay as they are.