Its always a bit difficult to know what to make of a situation from a legal point of view from press reports as they are often colloquial and don't set out the (boring) minutiae of the contractual position.
However, from the reports I have read it seems that the transfer was concluded under UEFA/FIFA regulations but not did not meet FA/Premier League rules. On the face of it this may mean that he was Cardiff's player but that as it stood they couldn't have fielded him in an FA sanctioned fixture. Unless there was something in the small print this could have left Cardiff in a rather embarrassing situation. I suppose we'll have to wait and see....
Saturday, 19 January: Cardiff confirm their club-record signing of Sala for an undisclosed fee thought to be around £15m.
That evening, Sala says: "It gives me great pleasure and I can't wait to start training, meet my new teammates and get down to work."
Cardiff's chief executive Ken Choo, who is present when Sala signs, says: "I'm sure all Cardiff City fans will join me in that and we can look forward to seeing our record signing in a Bluebirds shirt."