Sala, the Premier League club's record signing, was on board the Piper Malibu aircraft which disappeared from radar over the English Channel near Guernsey on January 21.
He and pilot David Ibbotson have yet to be found. It is understood that Cardiff received a letter from Nantes on Tuesday, in which the French Ligue 1 club threatened to take legal action if the first scheduled payment of the fee is not made within 10 days.
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.
The plane was only discovered on Sunday and the Air Accident Investigation Branch (AAIB) has not identified the body spotted in the wreckage, which was found on the seabed.
Cardiff would prefer to establish all the facts surrounding the tragedy first.
There is no suggestion that the Welsh club will not meet their contractual obligations but their point of view is that the investigation should first be completed.
It is also reported that Bordeaux are entitled to a cut of the fee for Argentinian striker Sala, who was on their books from 2012 to 2015 before joining Nantes.
A decision has yet to be taken on whether to raise the wreckage to the surface.