The BBC said the brief prequel, which will not be published, will go up for auction at Waterstone's Piccadilly store in London next month. Sotheby's will run the auction.
Money raised from the sale of the so-called storycards, featuring short original works by famous writers, is to go to English PEN, which promotes understanding through literature, and Dyslexia Action, the BBC said.
A rare, unpublished Rowling work, "The Tales of Beedle the Bard," was sold for nearly $4 million at auction last year.
Gerry Johnson, managing director of Waterstone's, told the BBC: "'What's Your Story?' just gets more and more exciting -- all these brilliant works by amazing writers and illustrators, every one a small masterpiece, and now this -- the prequel to 'Harry Potter.' We never dreamed that J.K. Rowling would donate something so precious, and we're incredibly grateful."
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