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Well, for the most part Soul Eater takes place in the US (Death City is in Nevada) so the currency in Death City is Dollars. However because they travel all over the world for their missions, the currency used depends on location.

-Previous statement is unlikely to be entirely correct, as when Spirit pays for services at Chupa♡Cabra's he places a note that says 'Death (something)' in the lower left corner, and has a denotation of 1000 on it.