Selling wheat for a specified cash price payable in 30 days is not sarf merely because money serves as the price. Wheat is the sold object, whereas cash is the price, and ordinary sale rules must be examined.
By contrast, one monetary asset exchanged for another raises specialized currency-exchange questions. Contemporary currencies, accounts and constructive possession involve further modern jurisprudential analysis; a statement about gold and silver should not be applied mechanically to every electronic transfer.