Pick up game dispute regarding block/charge/no call

[ATXCoach]

Player A (Offense) drives the basket for a lay up. Player B (Defense) slides over to take a charge. Player A steps on Player B's foot a goes down. Player A does not contact Player B's body until he's clearly falling.

Is it a travel or block?

It seems obvious that if B is standing there (legally established) and A steps on his foot and falls with the ball then it's a travel.

I don't know how to articulate well the fact that B may still be moving and had A not stepped on B's foot then B would have clearly contacted A's body.

Is there a scenario where A's foot has more right to the spot on the floor than B's foot?

Thank