let off vs excuse

let off is not a noun while excuse is a noun.

Word	Noun	Adjective	Verb	Adverb
let off	No	No	Yes	No
excuse	Yes	No	Yes	No

As verbs, let off and excuse are synonyms defined as:

Other synonyms of let off include exempt, relieve.

let off (noun)	excuse (noun)
	a poor example
	a defense of some offensive behavior or some failure to keep a promise etc.
	a note explaining an absence

let off (verb)	excuse (verb)
grant exemption or release to	accept an excuse for
	defend, explain, clear away, or make excuses for by reasoning
	excuse, overlook, or make allowances for; be lenient with
	serve as a reason or cause or justification of
	grant exemption or release to
	ask for permission to be released from an engagement