Cardiff were happy enough that all was done and dusted to announce him as their player, now they are checking the paperwork to make sure its all ok. Whilst its probably not great that they are battling this out in public Cardiff owe the money to Nantes for what they agreed. With transfers these days whilst it may be £15m in total there will be instalments and potentially some of it due if he makes so many appearances or scores so many goals.
I can sort of see Nantes point of view in all this...Cardiff were happy to pay the fee agreed to take the player off them and were happy to make a big announcement they'd signed him then this happens and all of a sudden Cardiff are holding back on paying what they owe. Compared to the English Premier League the French league isnt awash with TV money etc so the smaller clubs rely on transfer money like this to keep them going. This is a massive amount of money for a club like Nantes.
Interesting side point to this....the flight was arranged by a football agent by the name of Willie Mckay after Sala spoke to Mckay's son who is on the books at Cardiff about getting a flight. Said son, Jack Mckay, has since signed for Chesterfield on loan and has been cleared to make his debut this weekend if selected.