Donald Parfitt, a forklift operator at Clays Ltd. in Bungay, eastern England, claims he found the pages in the parking lot as he was leaving work May 5.
Parfitt, 44, was arrested two days later after The Sun newspaper told police it had received a call from "a shifty-sounding man" offering three chapters of the book for $40,000.
Defense lawyer Richard Mann said Parfitt was a "decent, hardworking man" who regretted not returning the pages immediately.
"There was lots of talk about papers offering money for the new book and he thought this was an opportunity," Mann said.
The fifth book in J.K. Rowling's series about a boy wizard, "Order of the Phoenix" is set for a June 21 release.
Rowling's four previous Potter novels have sold more than 190 million copies around the world, and hundreds of thousands of copies of the new book have been ordered in advance.
Parfitt is due to be sentenced June 4 at Lowestoft Magistrates Court. Three teenagers arrested with him have been charged with receiving stolen goods.
The Sun also reported last week that two unbound, uncut copies of the novel were found in a field near the offices of the printer.
The newspaper said a walker gave the copies to the newspaper, which alerted authorities and returned the books to their publisher, Bloomsbury. Mann said Parfitt was not connected with that incident.