excuse vs let off

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

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

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

Other synonyms of excuse include exempt, relieve.

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

excuse (verb)	let off (verb)
accept an excuse for	grant exemption or release to
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