Cardiff City are refusing to make interim payments to buy Emiliano Sala from Nantes for £15m , claiming that the deal was not legally binding.
Cardiff are set to tell the world football's governing body Fifa that Nantes' conditions for completion of the deal were not fulfilled because Sala was not registered as a Premier League player.
The French club referred the matter to Fifa, who want Cardiff to submit their evidence by April 3.
The Argentine died when an aircraft piloted by David Ibbotson, who is still missing, crashed into the English Channel near Guernsey.
A Cardiff source said the transfer agreement stipulated - at the request of Nantes - that the Football Association of Wales and France's Ligue de Football Professional had to confirm the registration to both clubs by January 22, along with confirmation of the international transfer certificate being released.
The Bluebirds insist the terms of the contract maintains that if any parts of that arrangement were not confirmed, then the deal would be null and void.
It is thought the notifications clause was inserted because if the deal fell through, both Cardiff and Nantes would have had time to seek a new player before the January transfer window closed on January 31
BBC Sport has also learned arrangements for a signing-on fee did not meet Premier League rules and so had been rejected by the league.
A Cardiff spokesman would not comment on specific details but said: ''The club is aware of Fifa's request for a response by April 3, and is processing that accordingly. We have no further comment at this stage.''
Nantes say they completed all the necessary paperwork and have pointed out Fifa registered the international transfer certificate on January 21
They say they have been fully compliant with Fifa's rules.