fast buck vs net profit

fast buck and net profit both are nouns.

Word	Noun	Adjective	Verb	Adverb
fast buck	Yes	No	No	No
net profit	Yes	No	No	No

As nouns, net profit is a hypernym of fast buck; that is, net profit is a word with a broader meaning than fast buck:

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

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

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