Ms Sagan, 63, has been accused by a millionaire businessman, Andre "Dede la Sardine" Guelfi, of taking a pounds 900,000 payment to intervene with the late President Francois Mitterrand to rescue a business deal.
The author, who wrote her best known work, Bonjour Tristesse, when she was 19, has categorically denied the charge.
The alleged incident forms part of a tangled web of financial and political manoeuvring that surrounded France's largest company, the oil firm Elf, in the Eighties and early Nineties. The judicial inquiry into Elf has already led to the formal investigation of the former Socialist foreign minister, Roland Dumas, who is now president of the Constitutional Council, the third highest political office in France.
Mr Guelfi - known as Dede la Sardine because he made a fortune in the fish business in Morocco - has been questioned by two investigating magistrates about his part in the efforts by Elf to win drilling contracts from the newly independent, former Soviet state of Uzbekistan in the early Nineties. In a four-page deposition to judges Eva Joly and Laurence Vichnievsky, Mr Guelfi said that he was promised a 5 per cent commission on all profits if he arranged a meeting between the Uzbeki government and the then head of Elf, the former senior civil servant, Loik Le Floch-Prigent.
When doubts about Elf's activities began to surface in 1993, he feared that Mr Le Floch-Prigent might be fired, ending his deal. He told the judges that he asked Ms Sagan, a friend of Mr Mitterrand's, to intervene to save the job of the Elf boss. In return, she was promised half his share of the Uzbeki profits.
In his deposition, leaked to the newspaper Le Parisien, Mr Guelfi says Ms Sagan undertook the task and that he paid her nine million francs over a period of several months.
Mr Le Floch-Prigent did continue in his job for a while but was removed later that year and now faces multiple charges of corruption arising from Elf's dealings with French, German and African politicians. Elf did win permission to drill in Uzbekistan but failed to find any oil.
Ms Sagan's lawyers and friends say that Mr Guelfi's version of events is entirely false. She never received any money from Mr Guelfi, they say. At Mr Guelfi's request, she did speak to the late president about Uzbekistan but only to persuade him - successfully - that he should make an official visit there.Reuse content