What stands out to me is the last part of that article. Where it says that the person or people organising the flight have let Sala down, as it was their responsibility to check the pilot was qualified and able to carry him as a passenger.

I'd assume this is why we are holding off payment to Nantes. If Mark McKay is the agent for Nantes and he didn't check everything was in order then it looks like he's at least partly liable. If you're in charge of flying an expensive footballer from one place to another you need to be damn sure everything is in order.