What is the rule by which two currencies are judged to be different: is it the difference in value or the difference in the issuing country of the currency?

What matters in currency exchange is the difference in value between the two types, as stated by An-Nawawi. Accordingly, it is permissible to exchange Iraqi currency with a premium due to the difference in value between its two types. The fact that the state is one does not matter if the value of the two currencies differs.