So in this AU, instead of being sent to jail, Frank is enslaved and, after a brutal six months of re-conditioning, sent to be legally owned by his lawyer, Matt Murdock. (Maybe Foggy is ineligible legally for some reason?)

Frank is still the Punisher/still can kill people but he's now completely traumatized and finds himself viscerally scared of/unwilling submissive to Matt, who has no idea how to do this.