give back vs refund

give back is not a noun while refund is a noun.

Word	Noun	Adjective	Verb	Adverb
give back	No	No	Yes	No
refund	Yes	No	Yes	No

As verbs, give back and refund are synonyms defined as:

Other synonyms of give back include repay, return.

give back (noun)	refund (noun)
	the act of returning money received previously
	money returned to a payer

give back (verb)	refund (verb)
pay back	pay back