Per Wikipedia: Allied Military Currency ("AMC") was a form of currency issued by the Allied powers during World War II, to be issued to troops entering liberated or newly occupied countries, as a form of currency control.

Historically, soldiers serving overseas had been paid in local currency rather than in their "home" currency;[1] most cash drawn by soldiers would go directly into the local economy, and in a damaged economy the effects of a hard currency such as the dollar circulating freely alongside weaker local currencies could be severely problematic, risking severe inflation.[2] There were other problems as well; once dollars were circulating in a combat region, the opposing side could freely use its own stocks of dollars as currency, or acquire stocks for use elsewhere.[3] The high purchasing power of the dollar, and its easy transference back to the United States, also posed a significant incentive to black-marketeering.[4]

However, whilst the use of local currencies was effective where they were provided in cooperation with the local authorities, it was impractical in combat zones where the government might be either hostile, deliberately ambivalent, or simply non-existent. In these cases, the military authorities issued special "military currency", which was paid out to soldiers at a fixed rate of exchange and simply declared legal tender in occupied areas by local commanders.