Oh so it was that. She knows the Playboy POS will likely send spies to MC territory to see if she's suffering (he was likely the one who pushed the change up only knowing MC via old outdated rumors since he likely thinks he's just a hick worse then he is) and if he finds out she isn't, along with the fact MC isn't like the rumors and thrice the man and noble he'd ever was, he'd interfere to ruin MC out of spite!
While she is right that scum as he is that was HIS money to use, she's wrong in that she is the only guilt one in "misusing funds" since it's likely he wasn't paying his tributes properly. So he thought he'd kill two birds with one stone and frame her for HIS crime, such as the tributes were apart of what she stole from him. The fact a frontier barons butler was able to get the whole story means the royal family isn't fooled either. But do to how FL handled it to give the people immediate aid, they likely can't call him out and bring him to task over it.
I feel MC wants to show her that the loser is nothing without his connections, title and money since he obviously put two and two together from her comment that she only said what she said out of fear of pushing her problems onto him. Wouldn't be shocked if asshole gets brought to task AFTER MC asswhopping on him to assure MC doesn't get in trouble (Like that Evil Intergalactic Lord manga where MC POS father tried to SNITCH on him having an AI help him run his territory to the Prime Minister when his plot to try and take back the territory after ruining it and then pushing it on MC his son resulted in MC restoring and making it profitable. PM laugh and brought up the obvious fact of how stupid his plot actually was. MC was competent, actually did his job as a noble putting his failed attempts to be an Evil Noble aside, and most important unlike his POS father and equally bad Grandfather, HE paid his taxes and tribute full amount and ON TIME! He even paid the unpaid amounts left by the two losers over two generations! That would be showing the competent Nobles that they'll be "punished" for following their obligation if he was to put a loser who ruined his territory and always FAILED to pay his taxes back in charge of his SON'S territory just cause the lout who DID EVERYTHING TO ASSURE HE WAS UNCONNECTED TO IT SO IT WOULDN'T HAUNT HIM so he had no legal ground to stand on, he did later try to use one of his mistress illegitimate kid when MC went missing cause he got LITERALLY ISEKAI to seize the property, that lasted about as long as it took for MC to quickly solve the problem, conquere that world, meet his old life daughter that also got isekai and impart a valuable lesson without either KNOWING the truth of their old relation and coming back to yell at EVERYONE ELSE but the AI as the only reason the idiot bastard brother got so far was his fan girls among his knights were fighting over his STORED SEMEN so they can be the mother of his child, fight with his fiancée AND ignored the bastard child since he was the WEAKEST only really having the fact he shared SOME DNA with MC to try and take his property when everyone thought he was dead when the girls should've known better)