hard cash vs hard currency

hard cash and hard currency both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard cash	Yes	No	No	No
hard currency	Yes	No	No	No

As nouns, hard cash and hard currency are synonyms defined as:

Other synonyms of hard cash include cash.

hard cash (noun)	hard currency (noun)
money in the form of bills or coins	money in the form of bills or coins
	a currency that is not likely to depreciate suddenly in value