I think he'll be a good player for them. Still only 21 and basically restarted his career with Sunderland last season. He has great talent but needed to play regularly after his move. Spurs tried to shove him straight into Championship teams when they should have started at League 1.
He's a winger so I'm not sure why we would have had any interest. It's not Football Manager, you can't just adapt any winger into an attacking midfielder/striker.
I suspect he set them back nearer £1 Million given he was in the last year of his contract. Sure, there will be add on but not ones Sunderland are likely to pay.