Asking BU on a Check Swing

[njdevs00cup]

Hypothetical: FED rules. No one on, BU in A. Batter checks his swing on a 3 - 2 pitch. PU calls ball four and the ball goes to the backstop. Batter sprints out of the batter's box and the defense wants an appeal. Catcher picks up the ball and throws to first. At what point does the PU ask the BU, right away or at the conclusion of the play?

The reason I ask is because if the PU goes to the BU right away and he has a strike, he is answering the PU and making a call on a throw to 1B. If the PU waits, and the batter continues to sprint to 1B, he can be tagged out if the BU agrees the pitch was a ball and the batter/runner over runs 1B.