Haha, yeah, a delicate issue indeed.
I don't know a great deal about commercial law, but in general terms (as I understand it anyway) once you pay for something, and ownership of that thing is transferred (i.e. there is a contract of sale etc.), the purchaser is solely responsible for the thing.
So, if you buy a car from a dealer, drive it off the lot and straight into a tree, you are solely responsible for the car and the damage etc. The seller has no liability whatsoever, as they have jettisoned their legal interest in the car via the sale. If the car has been purchased on finance (i.e. monthly payments or whatever), the buyer will still be liable for making those payments.
That model does not map so well to football transfers though, because obviously we are talking about the "sale" of human beings (or at least their contracts) rather than commercial products etc. and because transfer deals are rarely made up of flat fees/monthly instalments etc...
Obviously the sensitivity of the whole thing means that this won't really be discussed for a while, but I can see there being some legal disputes between Cardiff and Nantes in relation to this as time goes on. If no money has been transferred between the two so far then things could get pretty ugly. Nantes will obviously validly be able to claim that they are entitled to the cash, as at the time Sala went missing he was officially no longer their player. Whereas Cardiff might raise a "we are not paying out for nothing, as we might as well just flush 15 million quid down the toilet" type argument.
Could get very messy. One hopes that both parties are insured in relation to this sort of thing, and that none of the above transpires. It's primarily a very human tragedy obvs, but there will be some corporate fat cats somewhere who are sweating over this.
