It is thought Nantes will begin legal action if they do not receive a payment within 10 days.
The Piper Malibu N264DB carrying 28-year-old Sala and 59-year-old Mr Ibbotson disappeared over the English Channel on January 21 after leaving Nantes in France for Cardiff.
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.
Sala and pilot David Ibbotson were onboard a private aircraft, the wreckage of which was discovered on Sunday evening underwater in the English Channel.
However, Cardiff are yet to pay the first installment with the club still mulling over the details of the contract especially regarding recent developments surrounding the tragic incident.
Although Nantes were entitled to the first instalment of Sala's fee by now, Cardiff have been left surprised by the demand given the circumstances and the timing.
Sala, who signed a three-and-a-half year deal with Cardiff, was the club's record signing.
Authorities searching for Sala found a body in the wreckage of the plane, air accident investigators confirmed on Monday.
The situation is further complicated as French club Bordeaux are also due 50 per cent of the reported £15m transfer due to a clause inserted when Sala joined Nantes in 2015.