This is not a violation. Hit the whistle, kill the play, and give the ball back to Team B. The ball was never inbounded, the only thing you could do at all is give them a delay of game warning, but that's not even a good idea. The only way it would be a violation would be if it was a 5 second violation, which he has to step behind the line with ball in hand to do which he did not, or a boundary line infraction in which he steps behind the line with ball in hand and then steps over, I didn't say on the line, but over the line while ball is still in hand.
Man I remember the days when I used to get so screwed up with things like these. It's smart of you to ask about these small minute details
JS 20, cause as you move up in every level, the small minute details make a big difference. For Example, my boss is going to chew my butt when he sees my crews tape and sees that we had a violation happen in the backcourt and we gave the team the ball in the frontcourt. That is a big difference in our game and my boss will call me out on it. See what I mean. Minute, but a difference maker.