Pumping gas and forgetting your wallet happens from time to time and gas stations are or should be prepared for this situation.
I've forgotten my wallet before and I've also had my card declined before when paying for gas after I pumped it.
What I did was sign a form letting them know that I promised to come back and pay for the gasoline that I pumped and I wrote my tag number down to my vehicle, address and drivers license number.
I then went immediately to the bank and got the cash out and went back and paid them within 30 minutes of having the card declined.
The sooner you go back and pay the more they will trust you should it happen again.
If you forget your wallet go into the store and tell them you forgot your wallet and will either have to go to the bank to withdraw some cash or go home and get your wallet.
If you're from out of town try going to a bank and getting some cash from a bank account if you have your branch of bank in the area and have money in the bank.
If not promise to return at least the next day to pay for the gasoline once you found you wallet.