Fulham midfielder Jean Michael Seri, who is currently on loan at Galatasaray, has filed a criminal complaint of fraud against Nice relating to his move to Fulham in July 2018.
Seri joined the Craven Cottage club along with Maxime Le Marchand for a combined fee of around €25m.
The breakdown of that fee given at the time - according to Seri's lawyers was €12m for Seri and more than €12.6m for Le Marchand.
However, the midfielder claims the Ligue 1 club undervalued him in order to avoid paying him a "transfer premium" bonus and says he is owed at least €700,000, while insisting that some of his previous clubs would also be entitled to damages due to various sell-on clauses and a FIFA solidarity tax, which would take the figure to more than €1m.
His lawyers claim that, according to a report by the International Centre for Sports Studies (CIES) Football Observatory, if Fulham paid nearly €25m for the two players then €18.7m would "correspond to the transfer value of Mr Seri" with the other €6m to Le Marchand's.
A statement from his lawyers said: "According to the information available to Mr Seri, setting up a 'double transfer' to Fulham FC OGC Nice was thus able to grossly underestimate the amount of transfer due by Fulham FC with regard to Mr Seri and - correlatively - grossly overestimate the transfer amount relating to Mr Le Marchand.
"In the end, Fulham would have paid an amount of €12m for Mr Seri... and more than €12.6m for Mr Le Marchand. For those who know the transfer market, it is obvious that these numbers are quite strange.
"To illustrate the value that these two players represent in the eyes of Fulham FC, it is sufficient to point out that the salary granted to Mr Seri is approximately three times higher than that granted to Mr Le Marchand."
Nice have previously denied this claim and Sky Sports News have contacted Fulham for a response.
The French club are now owned by Ineos, the energy group run by British billionaire Jim Ratcliffe, who completed a takeover late last month.
Seri's statement added: "Mr Seri hopes that Ineos, which did not own OGC Nice at the time of the events, will be keen to shed complete light on this matter very quickly."