Richard Drysdale throws the baseball on Harlan's desk just to spite the dead Harlan. Blanc finds it outside then leaves in on a side table in the room they did the will-reading. Towards the end of the film, Linda finds the ball and places it back on her father's study. Where she finds the letter Richard discarded as blank shortly before throwing out the ball. That's how she finds out he's been cheating on her, leading to him getting a black eye, and probably a bad divorce (she made him sign a prenup) after the events of the movie.