Case 1: 6 saʿ wheat for 6 saʿ wheat, prompt exchange — equality condition satisfied.
Case 2: 6 saʿ wheat for 7 saʿ wheat, prompt exchange — prohibited excess.
Case 3: 6 saʿ wheat for 8 saʿ barley, present and identified — unequal quantities may be allowed because the genera differ.
Case 4: 6 saʿ wheat now for 8 saʿ barley next month — prohibited deferment.
One must check genus and delivery time in addition to quantities.
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Riba: Operative Causes, Unequal Exchange and Deferral
Case 1: 6 saʿ wheat for 6 saʿ wheat, prompt exchange — equality condition satisfied.
Case 2: 6 saʿ wheat for 7 saʿ wheat, prompt exchange — prohibited excess.
Case 3: 6 saʿ wheat for 8 saʿ barley, present and identified — unequal quantities may be allowed because the genera differ.
Case 4: 6 saʿ wheat now for 8 saʿ barley next month — prohibited deferment.
One must check genus and delivery time in addition to quantities.