This situation happened to me before. I prepaid for gas with $20 - which was all the cash that I had on me - and the pump continued another $5 or so before I noticed it and shut it off.
But I handled it differently. I went inside and told the clerk what happened. She apologized for forgetting to set the pump to auto shut off. I told her that I would be back later in the day on my way home from work and would make up the $5. Everything was fine.
I've since looked this up as a legal issue. If a store accidentally undercharges a customer for an item or service then the customer legally owes the difference. However, in the real world, when the clerk catches the error immediately, and requests extra payment, if the customer refuses (somehow justifying the windfall) then the store policy is to simply write off the loss if the amount is trivial. This is for two reasons. 1) Goodwill. Places of business don't last long with a policy of annoying customers over trivial issues. 2) Enforcement. The police and the courts will almost never agree to pursue an error for trivial amounts.
The bottom line is : The OP legally and ethically owed the extra 7 dollars. The idea that if the clerk forgets to set the pump to auto shut off and the customer receives more gas than he asks for then the customer gets the extra gas for free is baseless. If fact, every gas station everywhere has a sign on every pump that says the customer is required to monitor the pump and is responsible for all gas that comes out, including overages.
Of course, this story is complicated by the fact that the clerk was being snotty about it, but that doesn't change the monetary obligation. Then the clerk (allegedly) made a vague kind of threat to the customer in case of refusal to pay ("We're going to have a problem.") Maybe he was implying that he would call the police. Or maybe he was implying a physical confrontation. Or maybe something else. Personally, I would have asked him to explain exactly what he meant by a "problem" and then based my further actions on his answer. Regardless, the clerk was way out of line to put his hand on your car door to try to prevent you from leaving. Legally, this opens him up to assault charges. Just reckless.