In a letter to party activists, Jean-Francois Cope also acknowledged that there was strong evidence that the payments had "in reality served to meet the
expenses of the (2012 Sarkozy) presidential campaign".
Cope announced his resignation Tuesday in response to claims that €10 million ($13.6m) spent on Sarkozy's 2012 bid for re-election as French president were fraudulently billed as party expenses.
France's Socialist government on Wednesday demanded full disclosure of the former president's campaign financing.
"In a presidential campaign there are clear rules about financing limits, so it is important now that a full disclosure is made," said government spokesman Stephane Le Foll.
The scandal has left Sarkozy in what one observer described as a "lose-lose" situation.
The events-PR company which presented the contested invoices, Bygmalion, was run by two close associates of Cope.
He was ultimately responsible for the payments, as party leader, but claimed not to have been personally aware of them, saying: "Otherwise I would have blocked them."
At the heart of the scandal is the question of whether the bills were falsified to help Sarkozy or for some other reason, such as financial gain.
To date, Sarkozy has not been implicated directly in the scandal. But it has nonetheless been interpreted as another blow to his chances of staging a comeback in time for the 2017 presidential contest.
Thomas Guenole, the author of a book on Sarkozy's post-presidential career, said it was hard to believe that he could not have been aware that his campaign had been bolstered by additional funds equivalent to around 50 percent of the maximum he was legally permitted to spend.
"If he knew, he cheated on a massive scale. If he didn't know it raises a question of his competence," Guenole told AFP. "Either way, in terms of image, it is not good news."
Nathalie Kosciusko-Morizet, Sarkozy's spokeswoman during the 2012 campaign, said her then boss had been subject to such scrutiny it was inconceivable that
he had overspent so spectacularly.
"But it is easy to see what the interest of others could be and why they are now looking for someone to blame," she said. "Where did the money go? It is the people who run Bygmalion who can say."
Although Sarkozy remains a popular figure amongst party activists, his chances of running in 2017 have been eroded by the drip-drip effect of a series of scandals which have swirled around him since he left office.
His 2007 campaign is alleged to have benefited from funds provided illicitly by former Libyan dictator Moamer Kadhafi and France's richest woman, L'Oreal heiress Liliane Bettencourt.
Sarkozy was cleared last year of taking Bettencourt's money when she was too frail to know what she was doing, but his campaign treasurer is awaiting trial in that case and the Kadhafi probe also continues to rumble on.
The commission that oversees election spending last year found Sarkozy guilty of a relatively modest overspend of €466,000 and ordered him to repay some €10 million of public financing as a result.