I've never before seen any source citing it as 2500 or 4000, and am surprised to find so in your link.
HP wasn't published in the states until 1998. The 1997 is for the UK, sold (I think) in 1996, but a year between selling/publishing is the norm. I'm getting mixed up as to where you're referring to what; a potter fan is probably needed to sort out this dispute.
Ergh. I hate myself for getting drawn into semantics here.
lol, I can stop at any point, I promise.
Here's a link to an article on bloomberg.com discussing the amount:
https://www.bloomberg.com/news/articles/2005-05-29/nigel-newton
To quote:
"One of Newton's favorite maxims is "out of uncertainty comes discovery." But the chairman and chief executive of Bloomsbury had no inkling what a gold mine he had discovered when he offered J.K. Rowling a £2,500 advance ($4,700 at the current rate) for her first Harry Potter book. In fact, Harry Potter and the Philosopher's Stone, and its four successors went on to earn more money within the next decade than just about any book except the Bible. "We hit it lucky," says Newton, whose accent hardly betrays his California upbringing."
The 1,500 amount seems to come from an article located here:
http://www.scotsman.com/lifestyle/culture/books/the-jk-rowling-story-1-652114
This article claims 1,500. Clearly, there is a discrepancy. Which is correct, I have no idea, but I lean toward bloomberg.com.
Regarding the dates, I'm fairly sure they're accurate. The first advance was received in October of '96. The U.S. rights were picked up in the spring of '97, and the book was first published (as the philosopher's stone in the uk) on June 26, 1997. I haven't found anything really conflicting with those dates. After the release of the novel, it began to win awards and snowball from there.