I've had that happen to me before when I pumped the gas and went inside the store to pay for the gas and my card declined for some reason that I wasn't aware of at the time.
It happened to be a fault at the banks end which they fixed soon after and then I went and paid after I went to the bank and got some cash out.
What I did was sign a paper saying that I promised to be back to pay for the gasoline and gave them my address and tag number to my vehicle.
I then immediately went back to the bank and got the cash out because I knew I had the money in my checking account.
Then I went right back about 30 minutes later and paid the amount of money I owed.
It's not a crime to pump gas and forget your wallet or have your card declined and the gas stations will almost always work with you on the problem.
So just sign the paper they have you sign and pay the money as soon as you can.