They came from the gas station to check the meter. I have one cube less than...?

If you paid in advance, there is no problem. You can make an overpayment. The money remains on your personal account and will be debited only for actual gas consumption in subsequent periods. Overpayment is made only in monetary equivalent, not in cubic meters of gas. In case of overpayment, you must also transfer the meter reading to your timing operator.