Argentine striker Sala, along with pilot David Ibbotson, was on board the Piper Malibu N264DB which lost radar contact near Guernsey on 21 January.
Sala, 28, is Cardiff’s record signing but never played for the club.
Cardiff have withheld the first scheduled payment until they are satisfied with the documentation.
The transfer fee is due to be paid in instalments over three years.
In a later interview with French newspaper L’Equipe, Cardiff chairman Mehmet Dalman indicated that Nantes had sent an invoice for the first instalment, worth 6m euros (£5.27m).
In the same interview Dalman added: “We must show respect to the family. There is the process of recovering the plane.”
On Thursday night the Air Accidents Investigation Branch (AAIB) said a body had been recovered from the wreckage.
A source at Cardiff says they will honour the contract but not until they have clarified “all the facts”.
It is unclear whether or not the club have insurance covering the cost of the transfer.
And Cardiff are “surprised” Nantes have made the demand while attempts are being made to recover a body from the plane that was carrying Sala and Ibbotson.
The body was seen in the Piper Malibu on Monday, two weeks after the plane vanished near Guernsey.
French club Bordeaux are also entitled to a cut of the fee, thought to be 50% – Sala was on their books from 2012 to 2015 before joining Nantes.
The plane carrying Sala and Ibbotson, from Crowle, Lincolnshire, disappeared en route to Cardiff after the footballer returned to Nantes to say goodbye to his former team-mates.