Sala and pilot David Ibbotson were onboard a private aircraft, the wreckage of which was discovered on Sunday evening underwater in the English Channel.
A spokesman said: "Tragically in video footage, one occupant is visible amid the wreckage".
Although Nantes were entitled to the first instalment of Sala's fee by now, Cardiff have been left surprised by the demand given the circumstances and the timing.
A legal letter from Nantes was emailed to Cardiff on Tuesday - the second demand in the space of a week - to request urgent payment.More news: Trump's comments rile Iraqis, prompt troop pullout calls
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The Sala family are now in Nantes anxiously awaiting the fate of Emiliano and are still to learn whether it is even his body in the aircraft.
The club's insurance is believed to only cover around half of the total costs for the transfer, which would also include agent's fees and the player's salary over the course of his three-year contract.
The Air Accidents Investigation Branch (AAIB) are now trying to recover a body from the wreckage, although there is no confirmation whether the body identified is Sala's.
Bordeaux will also receive a percentage of the fee, thought to be 50 per cent, because Sala played for them from 2012-2015 before signing for Nantes.
An AAIB spokesman said, per the Evening Standard: "We are attempting to recovery the body".