Richard Ferrand, a minister in Emmanuel Macron's government, rented office space from his own partner for a fund he managed, it has emerged.
The revelation comes as Mr Macron's government prepares a law to tackle political corruption.
Both Mr Ferrand and the government say there was nothing illegal or unethical in the deal.
No public money appears to have been involved in the deal, which took place when Mr Ferrand was head of a health insurance fund, the Mutuelles de Bretagne.
However, tackling corruption in public life was a major campaign pledge from Mr Macron, who opposed MPs working as consultants or employing family members. The new justice minister is also currently drafting a bill on ethics in public life.
The allegations were first unveiled in the satirical French newspaper Le Canard Enchaîné.
In its report, it said the organisation led by Mr Ferrand had decided to rent office space from a building his partner was involved in.
On the basis of that guaranteed rental income, she was then able to secure a business loan worth about €402,000 (£347,000), the paper reports. Renovations carried out by the fund would also have increased the property's value, it added.
But Mr Ferrand was quick to address the issue on national television, saying the decision was made with full knowledge of his connection to the building.
"The administrators of the board, on which I did not sit, picked the best offer... which was a building owned by my partner," he told French broadcaster BFMTV.
He also said the buildings were still being used years later - indicating satisfaction with the arrangement.
He described the story as a "pseudo-scandal" and a "welcome present" for the beginning of his public life.
A government spokesman, Christophe Castaner, said that while the disclosure comes as a "bad time", there was no question of the minister resigning, or any suggestion of illegality.
Le Canard Enchaîné also alleged that Mr Ferrand's son had worked as a parliamentary assistant in 2014, which the minister dismissed as minor work which lasted for only four months.
Mr Ferrand has been appointed by President Macron as minister for territorial cohesion - a job he was picked for after helping the president to electoral victory as secretary general of the political party La République en Marche (Republic on the move).
Financial dealing between family members has become a political hot potato in France since the revelation last year - also by Le Canard Enchaîné - that presidential hopeful Francois Fillon had paid his wife for parliamentary work.
Media reports questioned whether she had actually performed the work she was paid for, and the controversy damaged his campaign, arguably clearing the way for Emmanuel Macron's victory.

The story started trending on Twitter, so we asked big-boned felines on the social network to not only show off their claw-some body shapes but also to give their views on the pay disparity.
This is, of course, just a tongue-in-cheek look at the serious issue of pay discrepancy - but if you feel you have any pictures that would help us encapsulate stories making the news, then do get in touch and tweet @BBC_HaveYourSay.
A close ally of France's new president is facing accusations over a property deal involving his partner.
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It is "Fat Cat Tuesday", the day of the year by which, according to campaign group the High Pay Centre, the average FTSE 100 chief executive will have earned more than the average annual salary.