A 24-hour petrol station uses pre-payment. This means that your incard is queried by VISA before you take on petrol according to a fixed amount defined by the merchant (fixed amount up to £120 or 150€, different from the future amount of your purchase).
This is the reason why the petrol pump displays a maximum payment amount.
Therefore, the transaction may be refused for insufficient balance, even if the amount of your purchase is less than the pre-payment.
If your business is in the UK and you would like to know more, please read the documentation on the VISA website.