Fuel Cards - Recorded on Till

Trying to figure this out!

Have been told that fuel cards allow a customer to purchase their fuel at a price lower than on the forecourt i.e. price set by fuel company etc. So, forecourt may say €1.40/ltr while charge by Fuel Card may be €1.30/ltr (example only, don't get excited).

How does the till record this i.e. a sale has gone through, similar to a Credit Card, but the price is less than quoted on the forecourt - can the till record the lower price? I assume it can't because the price is known by the card provider not the shop owner - the shop owner merely gets a commission?

Does this lead to a situation where the sales recorded on a till are overstated - leading to a cash difference and slight overstatement of vat on sales?

If anyone could shed any light, bugging me!