For everyone wondering, they're referencing an actual thing that happened during the American civil war.
Greenbacks were emergency paper currency issued by the United States during the American Civil War that were printed in green on the back. They were in two forms: Demand Notes, issued in 1861–1862, and United States Notes, issued in 1862–1865. A form of fiat money, the notes were legal tender for most purposes and carried varying promises of eventual payment in coin but were not backed by existing gold or silver reserves.
This was on a translation hiatus for like 3 months because of y’all grabbing this from the website. @swordmaster you need to stop, the team has threatened to stop translating it if people keep stealing. Get annoyed by the watermarks. Go support them on their website