Ms Bettencourt's former accountant told police she handed over €150,000 she was told would be passed on to Mr Sarkozy's campaign treasurer.
In July, a magistrate ordered the seizure of Mr Sarkozy's diaries, including his calendars.
The sum shocked French citizens as spending on political campaigns is tightly limited. Individual contributions to candidates are limited to €4,600, and no candidate can spend more than €22m on a presidential campaign. The French government reimburses some of that money to the winner.
Mr Sarkozy's election loss in May cost him his immunity from prosecution.