Higher market value is different from equality of weight. An old gold coin may have collectible value, yet such value does not simply remove the weight-equality rule for exchanging gold against gold.
Thus an 8-gram old gold coin cannot be directly bartered for 10 grams of gold merely because its market valuation is higher. Contract type and classification of both considerations must be established.
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Monetary valuation does not replace weight
Bab al-Sarf: Precious-Metal Exchange, Possession and Composite Goods
Higher market value is different from equality of weight. An old gold coin may have collectible value, yet such value does not simply remove the weight-equality rule for exchanging gold against gold.
Thus an 8-gram old gold coin cannot be directly bartered for 10 grams of gold merely because its market valuation is higher. Contract type and classification of both considerations must be established.