The 28-year-old Argentine forward was killed when the small plane carrying him came down in the Channel on January 21, two days after he completed his transfer from the French Ligue 1 side.
The Daily Telegraph said Sala signed a contract with Cardiff that was rejected by the Premier League and died before a revised one could be signed, adding that there is a dispute about whether he had agreed to sign a new deal.
According to the report, a source close to Cardiff said the agreement stipulated that Welsh and French football authorities had to confirm to both clubs that Sala “has been registered as a Cardiff City FC player and that the player’s International Transfer Certificate has been released”.
The source said this had not happened before Sala died.
“The transfer agreement between Cardiff and Nantes was subject to several conditions,” the source told the paper.
“If any were not satisfied, the contract would be deemed null and void, with no payment due. Nantes proposed that clause. They asked for the strict notification requirements.”
Nantes reportedly plan to dispute Cardiff’s case, saying the club completed the required paperwork. The club last month referred their dispute with Cardiff over the transfer to FIFA, football’s governing body.
Britain’s Air Accidents Investigation Branch (AAIB) previously said the plane carrying Sala, piloted by David Ibbotson, did not have a commercial licence.
But it said the journey would have been allowed as a “private” flight in which costs are shared between pilot and passenger.
It added that the basis on which Sala was a passenger had not been established.
The investigators also said since the pilot’s logbook and licence were not recovered, it was unclear whether Ibbotson was authorised to fly at night.
Sala’s body was recovered from the wreckage early last month but Ibbotson’s body has not been found.