How do i cancel a pending purchase in the App Store?
My experience is that the transaction doesn't take place at the moment you authenticate it. 

The download of an app commences, sure, but they don't necessarily take any money out of your account. I saw a week pass until I was invoiced and I'm wondering the same thing the person asking the question is, as I "bought" multiple apps when I had money in my account, then went out the next day and spent what I thought I had left. 

I am now constantly being asked to verify my payment information a week later as they never took the money out in the first place, and have restricted my use of the App Store to merely browsing the apps, I can't download or update any of them and they won't allow me to cancel it, it's ridiculous and their customer service is appalling.