ANSWER:

When you are a known person and you tell people you forgive them for whatever evil they had said about you, you encourage others to do the same and then ask for forgiveness. 

This was known from some of the scholars in the past whenever people would ask them for forgiveness to decline that and say to them: I would not make halal what Allah has made haram by allowing you and others to backbite and then forgive you. 

QUESTION: Please don’t take the question in a wrong way sheikh.I trust your knowledge that’s why I wish to learn.I’m just asking.When one asked you “Can you forgive me for anything I said behind your back ” you said “No, I can’t justify people backbiting and sinning and say that this is ok.”.But sheikh shouldn’t we forgiive and anticipate Allah’s reward?What was your wisdom behind the answer.