An occupying force prefers to use a money that 1) doesn't dollarize the nation they are occupying 2) can be tracked, thus thwarting black marketeers 3) is lightweight 4) if they lose, the enemy can't reuse it
Unfortunately, dollar bills are bulky, infiltrate local economies, and aren't enemy-resistant.
So we get innovations like EagleCash, which solves a lot of these problems.
Other payments ideas emanating from the military include Military Payment Certificates (https://jpkoning.blogspot.com/2017/04/c-day-and-military-mon...), overstamped currency ( https://twitter.com/jp_koning/status/1343874636480729088) and pogs (http://numispedia.org/AAFES/).