petty cash vs monetary fund

petty cash and monetary fund both are nouns.

Word	Noun	Adjective	Verb	Adverb
petty cash	Yes	No	No	No
monetary fund	Yes	No	No	No

As nouns, monetary fund is a hypernym of petty cash; that is, monetary fund is a word with a broader meaning than petty cash:

petty cash: a small fund of cash that a firm keeps for the payment of incidental expenses
monetary fund: a reserve of money set aside for some purpose

Other hypernyms of petty cash include fund.

petty cash (noun)	monetary fund (noun)
a small fund of cash that a firm keeps for the payment of incidental expenses	a reserve of money set aside for some purpose