وعليه ما يباع بنصف درهم من الفلوس
Translation: He owes the quantity of fulus ordinarily sold for half a dirham.
A sale price must be knowable, but a well-established customary quantity can provide that knowledge without stating the coin count. If half a dirham regularly purchases 12 fulus, 12 are due, not ten or twenty.
Where no determinate local convention exists, this particular reasoning cannot by itself remove uncertainty about the agreed price.
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What quantity of fulus is owed?
Bab al-Sarf: Circulating Fulus, Obsolete Coins and Money-Changer Exchanges
وعليه ما يباع بنصف درهم من الفلوس
Translation: He owes the quantity of fulus ordinarily sold for half a dirham.
A sale price must be knowable, but a well-established customary quantity can provide that knowledge without stating the coin count. If half a dirham regularly purchases 12 fulus, 12 are due, not ten or twenty.
Where no determinate local convention exists, this particular reasoning cannot by itself remove uncertainty about the agreed price.