What is the valid standard for future delivery contracts when the standard method is customarily disregarded?

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The soundest position is that a contract of salam is valid regarding the specification of quantity by whatever measure serves the purpose of defining the quantity, removing ambiguity, and preventing dispute upon delivery. This applies provided the condition of equality required in usurious transactions (ribawi transactions) is not violated. Commodities like grains, dates, raisins, pistachios, hazelnuts, and salt are fundamentally measureable by volume (kayl). Oils are also included as measureable items.