It is where the ball is at time of contact by player. If the fielder contacts ball while he is in fair territory but the ball is in foul territory, it is a foul ball. In your case, if the ball passed the bag fair and player knocked it foul, it is a fair ball.
OK, maybe I shouldn't have said anything about the dems. But Yobama happens to be one, and he is doing his dead level best to turn this great nation into a socialist one.