Munich-based Blanvalet Verlag snapped up the rights in February to "The Cuckoo's Calling", a purported debut novel by a certain Robert Galbraith about a private detective who investigates a model's suicide, its editor Anja Franzen told news magazine Focus.
That was five months before Rowling, the British author behind the mega-selling Harry Potter fantasy books, was unmasked as the novel's actual writer.
Focus said Blanvalet had paid a four-figure sum and was now counting on a bestseller, noting that the rights to Rowling books typically cost millions of euros.
Blanvalet, a Random House imprint, is now preparing an initial print run of 200,000 copies and will be released in German-speaking Europe "as soon as possible".
"I never suspected it could come from Rowling," said Franzen, adding that she had been fascinated by the novel's characters when she first read it.
Britain's Sunday Times newspaper exposed Rowling as the book's author this month after suspicions were raised at whether a first-time author could have written such an assured first novel.
The book is published in English by the Little, Brown Book Group which also released Rowling's first adult novel last year, "The Casual Vacancy", which drew far more mixed reviews.
The seven Harry Potter books, which were also made into blockbuster Hollywood movies, made Rowling a multi-millionaire.