The Argentine striker was the club's record signing.
It is understood Nantes are threatening legal action if they do not receive a payment within 10 days.
Cardiff are said to be "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.
Investigators have not been able to recover the aircraft which was flying from Nantes to Cardiff after Sala transferred from the French city's team to the Welsh capital's Premier League club.
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Queensland police assistant commissioner Peter Crawford said the man had suffered a "health issue" during the charging process. The incident ended when police shot the man with non-lethal beanbag rounds, after which he surrendered to the officers.
Cardiff's first payment would ordinarily be due within seven days of the player signing, but the club do not intend to make any payments until the conclusion of official investigations into the causes of his disappearance. BBC have attempted to speak to Nantes for a comment.
Cardiff are reported to be committed to honouring the deal but not until they have been provided with sufficient documentation.
Nantes, who play in France's top division, sold Sala to Premier League strugglers Cardiff in January for a fee reported to be £15million - although the fee is reported to be spread out over three years.
"In challenging conditions, the AAIB and its specialist contractors successfully recovered the body previously seen amidst the wreckage".
A recovery crew eventually found the aircraft in the ocean, with a body said to be visible in the wreckage.