Sala was signed by Premier League side Cardiff City in January for a club record fee of £15million, before the Argentine striker and his pilot went missing.
Pilot Dave Ibbotson was also on board as rescuers revealed one body has been spotted inside the aircraft 220ft under water.
Sala and pilot David Ibbotson were on board the Piper Malibu aircraft that went missing over the English Channel on January 21, carrying the player from Nantes to Cardiff ahead of his planned first training session with the team.
Now, Nantes is threatening to take legal action if they do not receive the payment within 10 days and expect Cardiff ot pay the full transfer fee.
Cardiff are "surprised" that Nantes have made the demand while recovery attempts are still being carried out.
A source at the Welsh club says the contract will be honoured but not before Cardiff have clarified "all the facts".
It is understood that the agreement between the clubs was to process the payments in instalments over three years.
Bordeaux, Sala's previous team, are thought to be due 50% of the transfer fee as per the terms of the deal which had taken him to Nantes.
Air accident investigators have confirmed that an attempt to recover the body found in the wreckage has begun.
"In challenging conditions, the AAIB and its specialist contractors successfully recovered the body previously seen amidst the wreckage".