The young striker who is currently on the youth team books had joined Villa from the London club without the Claret and Blue picking up the tab.
They have now been forced to pay an initial fee which could rise to as much as £1.2 million depending on appearances.
The club has high hopes for Forrrester and all Villa fans will be hoping the scouts have unearthed another gem to follow the likes of Gabby Agbonlahor and Gareth Barry.