Why not have a pre-paid fuel card for everybody? You top up in a petrol station, over phone, etc say put £50 on and then if you don't spend the entire £50 you can use it next time. Thse can be used in the Pay @ Pump machines and it only allows what amount you have on the card or less.
Companies can top up the cards and keep track of it and if someone wants to put £10 in they go into the shop and pay before hand!