Consider the situation: As a high-tier Goddess, Reality-chan has more authority than any government, and thus currency she issues is legal. The denomination of infinity also applies, since her panties are infinitely valuable due to being magic. The store can't simply ignore the infinite dollar bill, because word would get out, and the mafia, yakuza and the entire Chinese state army would rush to the scene and try to claim it, if not for its value, then for its magical properties. They can't destroy the bill either because Reality-chan's panties are indestructible.

Once the infinite-dollar bills fucks up the world economy, Reality-chan will administer a new, more functional system of currency, based around magic.