Pay £15M quid for an asset? Look after it. Don't just let it jump in a plane held together with stick-back plastic sub-contracted to the friend of a friend of an agent or whatnot.
Simply, this (although it won’t be that simple, clearly). Once he’d signed, he was Cardiff’s asset, and it was up to Cardiff to protect that asset - you’d expect through suitable insurance. As far as Nantes are concerned he’d been sold.
Only get-outs would presumably be any part of the fee based on performance-related aspects such as appearances, goals etc.
Quite possible only the lawyers are going to win here now (and the parasitic agents).