I saw in a recent thread about how it would take forever to get bitcoin to be accepted at the pumps of gas stations due to most pumps not having internet access.  It got me thinking, what if instead of a bitcoin credit card styled solution, there was a QR code at the pump with a wallet address, sending BTC to that QR through your wallet address would notify a server inside the store and turn the pump on for the amount of bitcoin you sent?

What if you wanted to pump until your gas was full, would there be a way for the pump to check the amount available before you pumped, so that it could pull it afterward?