The Haunted Man and the Ghost's Bargain

Redlaw is a teacher of chemistry who often broods over wrongs done him and grief from his past life. He is haunted by a spirit, who is not so much a ghost as Redlaw's phantom twin and is "an awful likeness of himself...with his features, and his bright eyes, and his grizzled hair, and dressed in the gloomy shadow of his dress..." This spectre appears and proposes to Redlaw that he can allow him to "forget the sorrow, wrong, and trouble you have known...to cancel their remembrance..." Redlaw is hesitant at first, but finally agrees. However, before the spirit vanishes it imposes an additional consequence: "The gift that I have given you, you shall give again, go where you will."
(Excerpt from Wikipedia)