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.
The Mirror reports that Nantes sent Cardiff an email last Thursday and followed up with a legal threat on Tuesday.
A search team found wreckage of the plane on Sunday in the English Channel, but bodies have not been recovered or identified.
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.
They are also keen to discover why, if as has been suggested, Sala's plane tried to take off on three separate occasions before beginning its final journey towards Cardiff.
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 plane carrying Sala, 28, and Ibbotson, from Crowle, Lincolnshire, disappeared on route to Cardiff on 21 January after the footballer returned to Nantes to say goodbye to his former teammates.
A spokesman for the Air Accident Investigations Branch (AAIB) said: "The AAIB is now considering the next steps, in consultation with the families of the pilot and passenger, and the police".