Reuven weighs 500 lbs. To say that he is so is loshon hara.
However, later, a blind person who heard my loshon hara is in a small elevator. The blind person hears Reuven say "Hold the door, I, Reuven, am coming to the elevator"
The blind person can then close the door and save himself from elevator failure.
Does the sin of me telling the blind person get erased because it later saved him from a loss?
If I told many people at the same time, does the entire sin get erased or only the portion that benefitted?
I made the person blind so he couldn't perceive reuvens size on his own.