The outlet reports Nantes will consider legal action if they are not paid "within 10 days", although Cardiff are said to be keen to clarify "all the facts" before honouring the deal.
Cardiff have yet to make a payment and Nantes put extra pressure on the Premier League side by asking for the first payment last Thursday.
The 28-year-old's body has not been found, although a body was...
Sky Sports News understands the club feels it necessary for the salvage operation in the Channel to be concluded and for both the Air Accidents Investigation Branch (AAIB) and the police to conclude their investigations before any payments are made.
But with the body not yet identified and Sala's family distraught, Cardiff is reportedly concerned about the timing of Nantes' demand.
Authorities haven't said if the body located by an underwater camera was Sala or pilot David Ibbotson, the only people on board.
Sala's plane disappeared into the deep blue waters while en route to Cardiff after saying his final goodbye to his former team-mates.
"Let's hope they can rescue it as soon as possible to find out what happened and have some sort of certainty", Sala's father Horacio Sala told the Associated Press Monday.
He added: "I communicated with them every day, but since I do not have WhatsApp it's hard to call them or call me".
"I can not believe it".