I'm not defending Crompton. Crompton looks like he doesn't know whether to scratch his head or crap his pants.
However, I watched the replay in slowmotion with my 20/15 vision, and Foster definitely dinged the top of the ball with his right forearm because he was not watching the ball in. Crompton softened his grip as he is supposed to during the transfer making it come loose easy, but it was in his hand up until the ball contacted the bottom of Foster's right forearm.
It was a bad exchange, but Foster is supposed to look the ball in, trap the ball and secure it. He was looking upfield.
Perhaps Crompton should have had a little firmer handle on the ball as well.