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