If the ball is in foul territory when the fielder first touches it then it is foul , but if the ball is in fair territory when the fielder first touches it, then bounces into foul territory it is a fair ball.

Any ball that strikes a defensive player while the ball is within the foul lines (i.e. in fair territory) is considered a fair ball... fielders are considered part of the playing field; hence, when José Conseco headed the ball over the wall in right centerfield, he was a part of the field (similar to a wall and not similar to the grass as a ball hitting the grass is no longer a fly) and the play was called a home run. If the player is standing in foul territory and reaches back into fair territory to attempt a catch and drops it, it is still considered a fair ball because fair or foul is determined by the location of the ball when first touched and not by the location of the fielder. I say when "first touched" because a fielder could conceivably bat the ball toward another fielder in foul territory with the second fielder also getting a "touch"... that would not matter as the ball was first touched while fair. Consequently, if the second player were to catch the ball and a tagging runner were to attempt to advance, he is free to do so as he can attempt to advance on a fly ball as soon as it is "first touched" by a defensive player. This keeps talented defensive players from "juggling" a ball all the way into the infield in an attempt to hold up runners on base and getting freebie outs by finally "misplaying" the ball then picking it up and achieving force outs at multiple bases.