Place the exchanged components into two correspondences:
First: 2 dirhams (silver) ← against 2 dinars (gold)
Second: 1 dinar (gold) ← against 1 dirham (silver)
Each correspondence is cross-metal, so equal number or weight is not its governing condition; mutual possession in sarf is still required. Simply noting that each party has three coins misses the legal reasoning.
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Mapping both sides without confusion
Bab al-Sarf: Mixed Currency Exchanges, Debased Coins and Obsolescence
Place the exchanged components into two correspondences:
First: 2 dirhams (silver) ← against 2 dinars (gold)
Second: 1 dinar (gold) ← against 1 dirham (silver)
Each correspondence is cross-metal, so equal number or weight is not its governing condition; mutual possession in sarf is still required. Simply noting that each party has three coins misses the legal reasoning.