dollar vs greenback

dollar and greenback both are nouns.

Word	Noun	Adjective	Verb	Adverb
dollar	Yes	No	No	No
greenback	Yes	No	No	No

As nouns, greenback is a hypernym of dollar; that is, greenback is a word with a broader meaning than dollar:

dollar (noun)	greenback (noun)
a symbol of commercialism or greed	a piece of paper money (especially one issued by a central bank)
a piece of paper money worth one dollar
a United States coin worth one dollar
the basic monetary unit in many countries; equal to 100 cents