paying back vs payment

paying back and payment both are nouns.

Word	Noun	Adjective	Verb	Adverb
paying back	Yes	No	No	No
payment	Yes	No	No	No

As nouns, payment is a hyponym of paying back; that is, payment is a word with a more specific, narrower meaning than paying back:

Other hyponyms of paying back include requital, retaliation, revenge, reciprocation, tit for tat.

paying back (noun)	payment (noun)
a reciprocal group action	the act of paying money
	an act of requiting; returning in kind
	a sum of money paid or a claim discharged