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The human has a terminal disease and the alien is incarcerated. The alien has a long lifespan and he wants to travel the universe. Hence, he exchanges "souls" with the human (without the human's consent). The alien couldn't handle paper with his bare hands because he was maybe hot or radioactive. So he sent metal sheets to the human, to write on with acid. I think it is a short story from a collection, written before 90s or early 90s.