A kind of Faustian story. The rat then, of course being the devil, and our protagonist being the one willing to offer a good friends life to write that one story. It is, at the bottom of it, about the choice. I think its rather clear that he never really believes that the rat is an usual kind of rat, in spite of all his efforts to call it a dream and not real and so on. So he makes the deal and, just as many who makes such a deal, thinks he has made a good choice. (he would die anyway) Good story, well told and interesting.