Cardiff City face a three-window transfer ban if they fail to pay Nantes the first instalment of the £15m fee for striker Emiliano Sala.
Sala, 28, died in a plane crash in January while travelling from France to join his new club.
Fifa has released the full reasons for its September ruling ordering Cardiff to pay the first 6m euros (£5.3m).
Cardiff will lodge an appeal against the decision in the next two weeks to the Court of Arbitration for Sport.
The Championship club have argued they were not liable for any of the fee because the Argentine was not officially their player when he died.
The world governing body’s players’ status committee (PSC) ruled Cardiff must pay the sum “corresponding to the first instalment due” in the transfer agreement and rejected the Welsh club’s arguments on a variety of grounds.
The PSC report reveals Cardiff had sought to delay Fifa’s judgement until possible civil and criminal proceedings, as well as an Air Accident Investigation Bureau investigation had been concluded.
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