Selling ears of wheat for an ordinary monetary price is not the same as exchanging wheat enclosed in ears for cleaned wheat of the same genus.
In a same-genus ribawi exchange, the amount concealed inside the ears can prevent verification of required equality and introduce a usury concern.
Therefore this general permission should not be read as allowing every unknown-ratio wheat-for-wheat exchange.
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Kitab al-Buyu: Grain in Ears, House Keys, Transaction Costs and Delivery
Selling ears of wheat for an ordinary monetary price is not the same as exchanging wheat enclosed in ears for cleaned wheat of the same genus.
In a same-genus ribawi exchange, the amount concealed inside the ears can prevent verification of required equality and introduce a usury concern.
Therefore this general permission should not be read as allowing every unknown-ratio wheat-for-wheat exchange.