Nantes are demanding the transfer fee Cardiff City owe them for missing striker Emiliano Sala.
The Premier League club have not paid the first instalment of the deal and say they will wait for the conclusion of any investigation into the 28-year-old Argentinian's disappearance.
In response the Ligue 1 side sent a legal letter to Cardiff City on Tuesday requesting the money owed to be paid within 10 days.
Cardiff were reportedly shocked by the timing of the letter from Nantes, as recovery workers are currently in the process of trying to excavate the body of someone from the crash plane he was in.
Sala was on a plane with pilot David Ibbotson travelling from Nantes to Cardiff on January 21 when the aircraft crashed into The Channel.
Cardiff spent a club-record £15m to secure the services of the striker who has played his entire professional career in France, having been at Bordeaux before joining Nantes.
The club will comply with the obligations when it comes to paying the transfer fee but are focused on ascertaining the facts around Sala's disappearance.
- Video report by ITV News correspondent Rupert Evelyn
The Piper Malibu N264DB carrying 28-year-old Sala and 59-year-old Mr Ibbotson disappeared over the English Channel on January 21 after leaving Nantes in France for Cardiff.
The plane had requested to descend before losing contact with Jersey air traffic control.
An official search operation was called off on January 24.
An operation to recovery a body seen in the plane wreckage has started.