quick buck vs earnings

quick buck and earnings both are nouns.

Word	Noun	Adjective	Verb	Adverb
quick buck	Yes	No	No	No
earnings	Yes	No	No	No

As nouns, earnings is a hypernym of quick buck; that is, earnings is a word with a broader meaning than quick buck:

quick buck: quick or easy earnings
earnings: the excess of revenues over outlays in a given period of time (including depreciation and other non-cash expenses)

Other hypernyms of quick buck include lucre, net, net income, net profit, profit, profits.

quick buck (noun)	earnings (noun)
quick or easy earnings	something that remunerates
	the excess of revenues over outlays in a given period of time (including depreciation and other non-cash expenses)