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.
The Premier League club have not paid the first instalment of the deal and say they will wait for the conclusion of any investigation into the 28-year-old Argentinian's disappearance.
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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.
The striker and pilot were on board the Piper Malibu N264DB aircraft which went missing more than two weeks ago between Nantes and Cardiff.
Sala's plane disappeared into the deep blue waters while en route to Cardiff after saying his final goodbye to his former team-mates.
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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.
The Telegraph reported a legal letter from Nantes was emailed to Cardiff on Wednesday (NZ time) to request urgent payment, which was the second demand in the space of a week.
And Cardiff are "surprised" Nantes have made the demand while attempts are being made to recover a body from the plane wreck that was carrying Sala and pilot Ibbotson.
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A recovery crew eventually found the aircraft in the ocean, with a body said to be visible in the wreckage.