A merchant uses one particular counterweight and agrees to supply dates equivalent to that weight for 300 rupees. The parties have accepted that fixed benchmark.
Replacing it later with a lighter stone changes the promised quantity. The weight need not have been translated into kilograms for the contract to refer to a definite standard.
This classical discussion does not displace applicable modern trade or weights-and-measures law.
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Kitab al-Buyu: Grain Sales, Bulk Heaps, Qafiz and Unit Prices
A merchant uses one particular counterweight and agrees to supply dates equivalent to that weight for 300 rupees. The parties have accepted that fixed benchmark.
Replacing it later with a lighter stone changes the promised quantity. The weight need not have been translated into kilograms for the contract to refer to a definite standard.
This classical discussion does not displace applicable modern trade or weights-and-measures law.