In the story, a guy with average or below-average intelligence gets a treatment that raises his intelligence considerably.
Then there was some sort of a conflict, possibly a blackmailing or trying to gain control of his employer. Maybe an accountant in a crime family, trying to become the boss? I can't remember the details clearly.
I think he might have failed, possibly got betrayed by his wife, who was cheating on him with one of the bad guys?
I read it in 2010s, in English, but most likely it was much older work. I don't believe it was longer than a short story or a novelette.
For some reason I used to think it was by Alfred Bester, but I couldn't find anything by him like this in my library.
I have already ruled out: Understand by Ted Chiang, Flowers for Algernon by Daniel Keyes