According to French radio station Europe 1, Sarkozy is due to be questioned on Friday by judge Jean-Michel Gentil, who is head of the inquiry.
Sarkozy was reportedly summoned to Bordeaux as part of the inquiry last Friday, but was unavailable.
The inquiry is to find out whether Liliane Bettencourt, who at the time was France's richest person, made contributions to the Sarkozy’s 2007 presidential campaign larger than the €7,500 legal limit.
Judge Gentil is set to question Sarkozy about a meeting between former finance minister Eric Woerth and Bettencourt’s accountant Patrice de Maistre in Neuilly sur Seine.
The meeting coincided with a sizable withdrawal from one of Bettencourt’s Swiss bank accounts.
Several people close to Sarkozy during the time of the alleged donations have also been questioned as part of the inquiry, including his lawyer Thierry Herzog, former secretary general of the Elysée Xavier Musca and former adviser to the justice department Patrick Ouart.