Several French media have published reports claiming Macron obtained huge discounts from venues and service providers during last year's presidential race.
“These reductions, in the order of 75 percent, must be brought to light,” said the Republican Front for intervention against corruption (Fricc), which is accusing the Macron campaign of “illegal financing”.
The group said there is an internal document of the national commission on campaign accounts which says it allows discounts or rebates of a maximum of 15 to 20 percent.
The head of that commission, Francois Logerot, stated on Thursday that its auditors had found no “irregularities” in the Macron accounts.
But Macron's critics have seized on the revelations given his emphasis on public integrity and accountability during his campaign.
The claims of financial favouritism come as Macron's chief of staff Alexis Kohler faces a conflict-of-interest inquiry over his links to the Swiss-Italian shipping giant MSC.
As a senior civil servant before Macron's election, Kohler worked closely on matters involving the shipyard STX France, where MSC is a major client.
MSC was founded by billionaire cousins of Kohler's mother, and Kohler himself joined the company as finance director in 2016 while continuing to work as an advisor on Macron's campaign.
On Friday, opposition Socialist Party leader Olivier Faure said France under Macron had become “the republic of cash”.
“A scent of permanent complicity reigns between the money centres and the executive. This republic which seeks to be an example has become a republic of cash,” he said.
“But you can't serve both,” he added. “You have to choose: the country or the cash.”