We have learned before that when someone speaks Lashon Hara about you, all of your sins are transferred to the speaker and all of his mitzvos are transferred to you. Quite the exchange!
However, halachah teaches that if someone asks you for forgiveness, you shouldn’t be cruel, and you should forgive them (see Shulchan Aruch 606:1). Therefore, if someone spoke lashon hara about you, but then asks for you forgiveness, you should forgive him.
The Ben Ish Chai zt’l asks: The lashon hara that was spoken against you redeemed you
from all your aveiros and gave you the speaker’s mitzvos. Who would want to forgive and lose all that good?! Wouldn’t it be wiser not to forgive the speaker of lashon hara, so you can remain with his mitzvos, and he remains with your aveiros?
The Ben Ish Chai answers that by forgiving your fellow man, all your sins will be atoned for, as the Gemara teaches “כל המעביר על מדותיו מעבירין לו על כל פשעיו” – Whoever forgives others, his sins are forgiven” (Yoma 23). So by forgiving, you will still remain without sins, so you may as well forgive your fellow man.
And although you will lose the mitzvos that you earned, nevertheless, forgiving is also a great mitzvah. Because by forgiving someone who spoke lashon hara about you, you are performing the mitzvah of Hashavas Aveida, returning lost items (because you are returning to him the mitzvos he lost due to his lashon hara). The reward for Hashavas Aveida is in accordance with the value of the item returned. So when one forgives, and gives back all the mitzvos he earned from the lashon hara, he will earn a great mitzvah of Hashavas Aveida. Therefore Shulchan Aruch advises that one shouldn’t be cruel, and he should forgive his fellow man, because by forgiving you don’t lose anything at all. ~ R’ Elimelech Biderman Shlita