Fifa are looking into the transfer payment for Emiliano Sala after Nantes made a claim against Cardiff City.
Cardiff are due to pay the first instalment of the £15m fee for the striker on Wednesday.
Argentine Sala, the Bluebirds’ record signing, died aged 28 in a plane crash in the English Channel on 21 January.
Cardiff were due to make the first of three instalments on 20 February, but the clubs agreed to extend the deadline by a week.
French side Nantes wrote to Cardiff on 5 February requesting for the first of three instalments.
But Cardiff said they were withholding payment until crash investigations were complete and that they were satisfied on “anomalies” around the deal.
His body was recovered from the plane wreckage but pilot David Ibbotson remains missing.