وعقد الصرف: بما وقع على جنس الأثمان
Translation: A sarf contract concerns exchange of assets belonging to the categories of monetary countervalues.
Sarf is a particular exchange where both countervalues are money-type assets, classically gold and silver. Buying grain or fabric with a cash price is not thereby a sarf contract.
Gold exchanged for silver is a basic illustration. The special sarf rule requires legally effective possession of both countervalues before the contracting session ends. The rules for ordinary sales cannot simply be transplanted into this exchange.