وَإِنْ كَانَ الْغَالِبُ عَلَى الْوَرِقِ الْفِضَّةَ: فَهُوَ فِي حُكْمِ الْفِضَّةِ.
Translation: If silver predominates in the coinage, it is governed by the rulings of silver.
The governing criterion is predominance. When silver remains the dominant substance, the mixed coin is treated under the silver nisab rules. A small admixture of base metal does not automatically turn it into merchandise.