1st time it happens (I assume this is a ball falling out of a pocket vs the server intentionally discarding the other ball) it is a let but the player who dropped the ball is issued a HINDRANCE WARNING. IF it happens again it is a point penalty. The same rule applies to a hat falling off ones head, Serena losing her beads, etc.
There is another scenario similar to this in which the server is holding 2 balls and then discards the 2nd ball if the 1st serve goes in:
USTA Comment 26.5: Can the serverís discarding of a second ball constitute a hindrance? Yes. If the receiver or an official asks the server to stop discarding the ball, then the server shall stop. Any continued discarding of the ball constitutes a deliberate hindrance, and the server loses the point.