hard currency 
noun [mass noun]
Currency that is not likely to depreciate suddenly or to fluctuate greatly in value.The government also announced that Cuban state companies would have to sell the central bank any hard currency received from exports or domestic sales....
- The Iraqi official said the money taken amounted to a quarter of the Central Bank's hard currency reserves.
- Their purpose is simple: to create a free trade environment conducive to the production of export goods in order to earn hard currency.