Ligue 1 side, Nantes have made the demand for a £15m transfer fee from Cardiff City on Emiliano Sala.
The Argentine striker, 28, was Premier League club Cardiff’s record signing but when missing after signing for the new club.
Sala, along with pilot David Ibbotson, was on board the Piper Malibu N264DB which lost radar contact near Guernsey on 21 January.
Cardiff has withheld the first scheduled payment until they are satisfied with the documentation.
According to BBC Wales has learned the transfer fee is due to be paid in instalments over three years.
It is understood Nantes are threatening legal action if they do not receive a payment within 10 days. The BBC has attempted to speak to Nantes for comment.
A source at Cardiff says they will honour the contract but not until they have clarified “all the facts”.
Concise News gathers, Cardiff is “surprised” Nantes have made the demand while attempts are being made to recover a body from the plane that was carrying Sala and Ibbotson.
The body was seen in the Piper Malibu on Monday, two weeks after the plane vanished near Guernsey.
French club Bourdeaux are also entitled to a cut of the fee thought to be 50% – Sala was on their books from 2012 to 2015 before joining Nantes.
The plane carrying Sala and Ibbotson, from Crowle, Lincolnshire, disappeared en route to Cardiff after the footballer returned to Nantes to say goodbye to his former team-mates.