Unfortunately, there isn't a good way to support this, because we can't control what the user does once they've already redirected. The best thing to do would be to place a hold on the stock set to expire after a certain amount of time - perhaps even a couple hours unless you're moving product very fast. But still, if the customer completes payment remotely, it's gonna get reported, and you'll just have to deal with it.

I'd establish a process along the lines of detect if the stock dropped below 0, and if so send a back order e-mail or else automate some way to void the transaction and notify the customer that their product has dropped out of stock. In fact, perhaps what you should do is simply authorize the transaction and wait until you confirm their payment to capture it so you don't have to issue a refund, just void an authorization.