Nantes sold a player to Cardiff. What happens after that point is irrelevant to Nantes, who are due the first payment installment. It makes no sense for Cardiff to withhold payment until any investigation is completed, because irrespective of the results of that investigation, Nantes will still be owed payment. Splitting the transfer fee is nonsensical on many levels. If Cardiff's insurance policy is in order (and it's their responsibility for it to be so) then they will receive an insurance payout that should cover some/all of their financial obligations for this transfer (depending on the insurance T&Cs, that could cover the transfer fee, Sala's wages, etc.).
The only dispute I can potentially see is between insurers. I assume the plane/pilot were insured, so depending on the terms of that insurance, Cardiff's insurance company may be able to go after the plane/pilot's insurance company if it can be proven that the plane had mechanical issues or the pilot acted recklessly in attempting to fly in the conditions, etc. (as would happen in a car accident involving two vehicles, for example).