Sarkozy is accused of accepting $150,000 from L'Oreal heiress Liliane Bettencourt, now 90, for his election campaign, BBC said.
Prosecutors said the former president is under investigation for taking financial advantage of a vulnerable person, Radio France Internationale said.
On Thursday, the Bordeaux court summoned Sarkozy, who has hinted he may take another run at the presidency in 2017, RFI said.
At the court appearance, the Bettencourt household staff testified that Sarkozy had visited the house several times. Sarkozy has said he only went there once.
Sarkozy's lawyer, Thierry Herzog, told RFI his client is offended by the claims.
"Nicolas Sarkozy is still combative but, at the same time, he considered the treatment inflicted on him scandalous," Herzog said Thursday.