The presidential candidate has spent the past week explaining that his wife worked fairly to earn around €900,000 over eight years as a parliamentary aide, first to Fillon and then to his fellow MP Marc Joulard.
His wife Penelope, who has said in the past that she prefers life in the shadows, has been splashed over the front page of every newspaper in the country in recent days.
And now, it appears that an old interview is about to come back to haunt her.
Envoyé spécial, a French investigative TV programme, claims to have ten-year-old footage of Penelope in which she denies working for her husband.
“I've never been his assistant,” she reportedly told the UK's Sunday Telegraph in 2007, which given Fillon has spent a week insisting she was, is now an explosive statement.
“I'm not doing his communication,” she was also said to have said on video to the Sunday Telegraph.
Penelope Filon was paid as her husband's parliamentary assistant between 1998 and 2002, until he became minister. After that she worked for Joulard, a friend of Fillon, until 2007.
Envoyé Special, which will be shown on French TV on Thursday night (Channel France 2 at 9pm) have managed to track the video down.
The footage was from a 2007 interview given to freelance journalist Kim Willsher, that was also filmed by a Sunday Telegraph photographer.
The quote probably did not appear in the original print article because at the time it meant little and there was no reason to doubt her, Willsher told The Local on Thursday.
In France it is legal to employ family members as aides but they must be actively doing the job.
One quote from Penelope Fillon that was included reads: “People ask what my new role is but there isn't one,” before she went on to stress how she preferred life out of the limelight at the family chateau in Sarthe.
Another old quote that has haunted Penelope Fillon this week is when she told French media in October last year that she had never been involved in the political life of her husband.
Fillon has since insisted she meant on the front line, while saying she has long worked in the shadows on his behalf.
Fillon has admitted to employing Penelope and two of his children during his time in parliament, with the newspaper Le Canard Enchainé reporting they earned pre-tax income of around €900,000 ($970,000) altogether.
Police have launched an investigation into the possible misuse of public funds, which the Fillons have cooperated with so far.
The allegations also surround Penelope Fillon's earnings from a literary review owned by a billionaire friend of her husband's where she allegedly picked up €100,000.
Whether or not he ends up in trouble with the law is not Fillon's only issue. It seems the public has already lost faith in him, with polls showing his popularity has tumbled and a new survey suggesting seven out of ten French people want him to stand aside.
If the programme's claims are true, the Fillons can probably expect the nightmare to get worse.