Fath al-Qadir records Zufar's objection: fulus are counted, so saying 'half a dirham's worth of fulus' without stating the number risks an unknown price.
The answer is that an established custom, under which the number purchased for half a dirham is commonly known, removes uncertainty about the consideration. The question is reliable determinability, not merely a rigid verbal formula.
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Zufar’s objection to an unstated coin count
Bab al-Sarf: Circulating Fulus, Obsolete Coins and Money-Changer Exchanges
Fath al-Qadir records Zufar's objection: fulus are counted, so saying 'half a dirham's worth of fulus' without stating the number risks an unknown price.
The answer is that an established custom, under which the number purchased for half a dirham is commonly known, removes uncertainty about the consideration. The question is reliable determinability, not merely a rigid verbal formula.