If he didn't control the ball until he was on the line, then A has secured team control in their backcourt: "ART. 2 . . . When a player is touching the backcourt, out of bounds or the three-point line, the player is located in backcourt, out of bounds,or inside the three-point line, respectively." Ergo, not a violation.
Let's make this more interesting. What if the A player had begun his slide and secured the ball in frontcourt, but the momentum of his slide took him across the line. Do we have a violation?