The probe into possible influence peddling is focusing on clothing furnished by tailor Arnys in February worth 13,000 euros (USD14,000) and paid for by an unnamed wealthy friend, the source told AFP.
The Journal du Dimanche newspaper claimed that an anonymous benefactor had paid nearly 48,500 euros for Arnys garments for the former prime minister since 2012, including the clothes paid for by cheque last month.
The other gifts were paid for in cash, the paper reported.
The revelations prompted Fillon to repeat assertions that he was the target of a "witch hunt" by journalists.
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He was charged Tuesday with misusing public money in respect of payments totalling 680,000 euros to his wife Penelope between 1986 and 2013 from funds made available to lawmakers for parliamentary assistants.
An avowed Catholic, who has campaigned as a man of integrity, has also been charged for failing to declare a 2013 interest-free loan of 50,000 euros from Ladreit de Lacharriere to a state transparency watchdog.