Permission for bulk exchange is not a general licence to exchange unidentified or absent goods. The two particular, present countervalues may be traded without matching their weights when the metal genera differ.
For example, exchanging a particular gold bar for a particular pile of silver in the same meeting does not fail for inequality of weight. Promised delivery after separation still raises the prohibited-delay problem.
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Limits of bulk exchange
Bab al-Sarf: Precious-Metal Exchange, Possession and Composite Goods
Permission for bulk exchange is not a general licence to exchange unidentified or absent goods. The two particular, present countervalues may be traded without matching their weights when the metal genera differ.
For example, exchanging a particular gold bar for a particular pile of silver in the same meeting does not fail for inequality of weight. Promised delivery after separation still raises the prohibited-delay problem.