The conservative candidate "did not deem it necessary" to report the 50,000 euros ($53,000) loan he received from Marc Ladreit de Lacharriere in 2013 to a state transparency watchdog, the Canard Enchaine weekly said in its edition to appear Wednesday.
"The 'oversight' may be costly for the presidential candidate," said the investigative and satirical newspaper, which also made the allegations in January about the fake jobs scandal that has threatened to derail Fillon's candidacy.
Le Canard Enchaine reported that Fillon's lawyer Antonin Levy had confirmed the loan had been repaid in full, but did not say when.
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French rightwing party vows 'unanimous' support for Fillon
Once the frontrunner to become France's next president in May, 63-year-old Fillon has had to battle to stay in the race because of the revelations that he had paid his wife Penelope hundreds of thousands of euros from public funds, allegedly for fake jobs.
The former prime minister is to be charged later this month.
Ladreit de Lacharriere is the CEO of Fimalac, a financial services holding company, and owns the literary magazine La Revue des Deux Mondes.
The publication paid Penelope Fillon some 100,000 euros in 2012-13 but there is little evidence of her work.
Investigators are looking into a possible link between this job and the bestowal of France's highest civilian honour, the Grand Croix of the Legion of Honour, on Ladreit de Lacharriere in 2011 when Fillon was prime minister.
The Canard Enchaine also said investigators were looking into a consultancy firm called 2F Conseil that Fillon set up in 2012 after he left office as prime minister, which the paper says has paid him hundreds of thousands of euros.
Fillon has denied any wrongdoing with his consultancy work.
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Party's 'unanimous' support
With just seven weeks to go before France goes to the polls in the April 23 first round of a two-stage vote, Fillon has remained as the rightwing candidate despite calls for him to allow rival Alain Juppe to stand in his place because of the scandal.
On Monday, he won the "unanimous" backing of his Republicans party after Juppe, 71, adamantly rejected the calls while lamenting the state of Fillon's candidacy, calling it a "waste".
Juppe also chastised Fillon for the disdain he has shown the justice system and his swipes at the media.
Fillon, campaigning as a sleaze-free candidate, was the surprise winner of the rightwing primary in November, besting Juppe who was convicted in 2004 over a party finance scandal.
The infighting among Republicans and Fillon's chaotic campaign have made an already unpredictable election even harder to call.
Centrist candidate Emmanuel Macron, 39, has gone from strength to strength, with the latest poll putting him neck-and-neck with far-right leader Marine Le Pen with 25.5 percent to 26.5 percent for her.
Fillon is lagging behind in third place with 18.5 percent, according to the Ifop-Fiducial poll out Tuesday.
