When a sale specifies a price but does not identify which kind of currency is intended, prevailing local practice may supply an interpretive clue.
If one currency overwhelmingly predominates in local transactions, the price expression may be interpreted by reference to that usage. It is a case of muʾawwal: customary evidence gives one meaning preference.
If different currencies circulate without a sufficiently dominant one and differ in value, the price may remain materially uncertain, creating a legal defect in the contract.
The point: credible predominance can guide interpretation; a party's later arbitrary choice cannot substitute for the missing agreement.