According to the BBC, Cardiff have withheld the first scheduled payment until they are satisfied with the documentations.
The Argentine striker had completed a transfer from the French club to Neil Warnock’s side before a plane he was travelling on went missing on January 21.
On board with pilot David Ibbotson, Sala’s plane went missing over the English Channel on his way to Cariff and debris from the aircraft is now being recovered
source at Cardiff says they will honour the contract, but not until they have clarified “all the facts.”
The transfer fee is due to be paid over three years and Nantes want the first payment this month.
Reports of Nantes’ demands come amid the discovery of a body aboard the wreckage of the plane.
Investigators have not confirmed if 28-year-old Sala is the body visible in a video footage taken underwater of the crash site.
A spokesman for the Air Accident Investigations Branch (AAIB) said: “The AAIB is now considering the next steps, in consultation with the families of the pilot and passenger, and the police.”