free agent vs doer

free agent and doer both are nouns.

Word	Noun	Adjective	Verb	Adverb
free agent	Yes	No	No	No
doer	Yes	No	No	No

As nouns, doer is a hypernym of free agent; that is, doer is a word with a broader meaning than free agent:

Other hypernyms of free agent include actor, worker.

free agent (noun)	doer (noun)
someone acting freely or even irresponsibly	a person who acts and gets things done
(sports) a professional athlete who is free to sign a contract to play for any team