Ferrand was revealed to have benefited from a property deal while he ran a public health insurance fund.
Richard Ferrand, a crucial early supporter of Macron and a senior minister in his first government, was revealed to have benefited from the deal while he ran a public health insurance fund.
The Canard Enchaine newspaper reported Tuesday that while working as director general of a fund in his native Brittany region in 2011 the organisation agreed to rent a building proposed by his partner.
On the strength of the rental contract, she secured a loan enabling her property management company to buy the building worth around 400,000 euros ($450,000), the report said.
Substantial renovation work was carried out by the fund which increased the property's value, it added.
"The administrators of the board, on which I did not sit, picked the best offer... which was a building owned by my partner," Ferrand told the BFM television channel on Wednesday.
The lawmaker, elected to parliament in 2012 as a Socialist, called it a "pseudo-scandal" and said it was a "welcome present" for the start of his time in government.
Macron, a 39-year-old independent, won France's presidential election on May 7 on the back of his grassroots movement called En Marche (On the Move) and has promised legislation to improve ethical standards in public life.
The timing of the revelations, which were widely covered in the French media, is unfortunate for him ahead of crucial parliamentary elections in June when he is seeking a majority to push through his reform agenda.
Ferrand faced calls Wednesday from far-right leader Marine Le Pen to resign, while the head of the Socialist party Jean-Christophe Cambadelis called on him to explain himself.
"On the question of him resigning, it hasn't even been discussed," government spokesman Christophe Castaner told a press conference. "The probity of the minister is not in doubt at all."
The health fund managed by Ferrand, the Mutuelles de Bretagne, is one of dozens of private health insurance funds which are a key component of the French public health system.
The state reimburses the majority of medical costs for users of the health system, with the rest covered by the private funds.