cancel purchase required!!

You cannot cancel the purchase as such: you would have to request a refund.

Strictly speaking all purchases are final (after all, you can't very well return them): please see this page for information on requesting a refund: Request a refund for apps or content that you bought from Apple - Apple Support

However Apple have been known to be sympathetic in cases of actual error, so it's possible that they may allow you a refund as a one-off concession: but please be aware that they are under no obligation to do so: go to  https://reportaproblem.apple.com/?s=6

Purchases are considered final as you know, but you could try the 'report a problem' page to contact iTunes Support and see if they will refund you as a one-off : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on ‘Get Help’)