first thing that must be checked out legally is:, public performance figures similar to the MMA fighters, generally "own" copy-right usage on their character images. your art is also based on already-published images of them.
so the first question is to find out whether their image use is copy-righted and, if so, will they allow use of it. If so, either each performer may require permission and fees. will they share any rights with you. ?
I don't know anything about MMA association. it is also possible that rather than copyrights per artist, the association managing them owns such copyrights and may give permission , and collect payment the same way ASCAP and BMI do for songs [the country’s two leading performing rights organizations (PROs) collectively represent songwriters, composers and music publishers]
Think of how Disney owns exclusive rights to their characters. Not just any old individual Joe can sell items with their images on them.
If the MMA characters are not well represented, this may differ for them.
If the performers or association are "hungry" enough, sharing profits could be appealing. but usually a quality-approval is also all connected to use.
I would not just hand it over to some squirrel to use. in the end, you share liability, as well as potential profits.
EVERYthing you have already posted in public domain is evidence, (if things will be "for sale") and so are your correspondences with this clown.
Why give permission to HIM, when you potentially have a larger market by working WITH the ones who own copyrights?
Good luck.,
Granny.