What are the conditions for specifying the countervalues in a liability-based currency contract?

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A condition for liability-based currency exchange is that both countervalues must be known, either by a defining characteristic or by recourse to the known or prevalent currency of the locality, in which case a general mention reverts to it. If one states, "I sell you an Egyptian Dinar for twenty Dirhams of the currency known as 'Ten per Dinar'," the sale is invalid unless that specific type ('Ten per Dinar') is the only known currency in the locality, in which case the general statement reverts to that specific type. The same ruling applies to sales transactions in general.