Is unintentionally seeing part of someone's major awrah via an electronic chat considered a sin, and what should be done to expiate it?

The questioner must offer sincere repentance for what he committed by agreeing to someone exposing their nakedness and looking at it, for exposing private parts and unleashing one's gaze upon what Allah has forbidden are abominations that incur Allah Almighty's wrath. He is also advised to choose carefully those with whom he converses to avoid undesirable consequences.