After a series of phone calls from the French club, Sky Sports News understands Cardiff received the first letter from Nantes' lawyers more than a week ago.
The move was seen as poorly timed given the effort to recover the 28-year-old footballer's body from the wreckage of the Piper Malibu plane which crashed into the English Channel is still ongoing.
A source at Cardiff told the BBC they want "all the facts" clarified before they honour the payment and that the club are "surprised" the demand has been made before Sala's body has been recovered.
The light aircraft was later discovered lying in the bottom of the English channel after days of endless search for the missing player and pilot.
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Cardiff and Nantes agreed a £15 million fee for Sala, due to be paid in three instalments, on January 19 but the Argentinian striker disappeared on a flight from Brittany to Wales two days later along with pilot David Ibbotson.
But with the body not yet identified and Sala's family distraught, Cardiff is reportedly concerned about the timing of Nantes' demand.
French club Bourdeaux will receive around 50 per cent of the fee as Sala played for the club between 2012-15 before joining Nantes.
The Air Accidents Investigation Branch (AAIB) are now trying to recover a body from the wreckage, although there is no confirmation whether the body identified is Sala's.