A search team found wreckage of the plane on Sunday in the English Channel, but bodies have not been recovered or identified.
Sala, 28, was travelling from France to join up with his new Premier League club in a light aircraft on January 21 when it went missing close to the Channel Islands.
According to a report by BBC Wales, Nantes have demanded receipt of the first instalment of the fee and have threatened legal action if it is not received in 10 days' time.
However, it is not clear whether the body discovered is Sala's or Ibbotson's. BBC have attempted to speak to Nantes for a comment. The two clubs had agreed on a payout in three instalments, which is relatively common in big transfers.
Cardiff are reported to be committed to honouring the deal but not until they have been provided with sufficient documentation.
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"In challenging conditions, the AAIB and its specialist contractors successfully recovered the body previously seen amidst the wreckage".
French club Bourdeaux are also entitled to a cut of the fee, thought to be 50 per cent after Sala was on their books from 2012-2015 before joining Nantes.
An AAIB spokesman said, per the Evening Standard: "We are attempting to recovery the body".
"Unfortunately, attempts to recover the aircraft wreckage were unsuccessful before poor weather conditions forced us to return the ROV to the ship".
In a statement, the Air Accidents Investigation Branch (AAIB) said: "Following extensive visual examination of the accident site using the remotely operated vehicle (ROV), it was made a decision to attempt recovery operations".