Nantes have begun legal action against Cardiff City, demanding the first payments for the purchase of Emiliano Sala, Sky Sports News understands.
It is understood the Ligue 1 club are demanding an initial payment of £5m is transferred immediately.
Cardiff have not yet paid any of the £15m fee for the Argentine striker, according to Sky sources.
Sky Sports News understands the club feels it necessary for the salvage operation in the Channel to be concluded and for both the Air Accidents Investigation Branch (AAIB) and the police to conclude their investigations before any payments are made.
Cardiff are happy to honour the contract agreed with Nantes but believe there are a series of questions that still need answering, most notably about the ownership of the Piper Malibu aircraft which crashed North of Guernsey on January 21.
They are also keen to discover why, if as has been suggested, Sala's plane tried to take off on three separate occasions before beginning its final journey towards Cardiff.
After a series of phone calls from the French club, Sky Sports News understands Cardiff received the first letter from Nantes' lawyers more than a week ago. The Premier League club's lawyers have responded, asking for more time.
At this stage, it is not known whether any legal case would be heard in France or the UK.
Sky Sports News have attempted to contact Cardiff and Nantes for comment, so far without a response.