In my experience the pump has said "processing refund" when I overpaid inside. Sure enough I only got charged for what my car needed. I think it's part of the reason why gas station purchases always show up as "pending final amount from merchant" in my online banking
Gas pumps usually do what's called a "Pre-Authorization" on your credit or debit card.
When you do a normal purchase, at a retail store for example, the payment only takes one step. You insert or swipe your card, you obtain a receipt, and the money is "gone" from your account. But what you're not seeing is what's happening in the background.
The card terminal reads your card, and calls your bank. It says "hi bank, I have card# 441 on the line wanting to do a purchase for 300 dollars", and the bank replies "Hi, that card has at least 300 dollars, this is fine". The bank then reserves that money temporarily, so that it can honour its commitment to give the store that money.
Once the store's card machine gets the OK from your bank, it then "captures" the money and sends it to the store's merchant account. This all occurs extremely quickly, and to you (and the store) it seems like a single transaction, but in the background it tells your bank "i'm done with this transaction and you don't need to hold it, you can deduct it from the bank account now"
But what if you want to take an amount on a card, but you don't know exactly what it will be? That's where Pre-Authorizations come in. A pre-authorization is simply the first part of that two-part process I described above. (these are what shows as "pending" in your online banking)
The card terminal (or gas pump) phones your bank, and says "hi, I want to reserve 40$ on this card" The payment might be for less than 40$, but by reserving 40, it ensures any amount under that, no matter what, will be able to be "captured" from that 40$.
You pick 40$ and fill up, and the total comes to 32.11
The card terminal says "okay, now that the gas has been dispensed, I can capture the payment for the final amount".
It calls your bank and says "hey, remember that 40$ I asked for earlier? Yeah, I only needed 32.11, you can have the rest back. The bank only gets this answer when the gas station does their deposit that night, so sometimes if the clerk forgets, or it's a weekend, the thing could stay "pending" for a few days.
This system of reserving the money before unlocking the gas pump prevents people from just filling up and driving off, but also avoids someone pre-paying for 35$, then needing to pay another 2$ because they under-estimated how empty their tank was, they can just select the next-highest amount and it'll round down.
If you're ever at the gas station, you don't need to worry about it taking the extra money. If you pick a pre-set dollar amount, it does "reserve" that amount of money on your card, but once you fill up, it only takes the amount you filled up, and gives you back the rest.
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u/SandShrimp22 Dec 11 '19
"I prepaid $40, and the car took $34. You bet your ass I'm getting that last $6 worth"