>>Does this mean the cardholder has bought fuel beforehand, which is now "stored" by the company but actually owned by the cardholder?
Sounds like it to me.
>>which would mean they take back the already bought but not used fuel as compensation for money owed by the cardholder
I don't think so. It looks like the reference here is to Cardholders. That is, Cardholders can get their money back (the "outstanding balance"), and the amount they collect "would be at the current average Company buying price." In other words, it wouldn't matter what the initial purchase price was. If you bought at price X, and the price has now risen to X plus y, the Cardholder would receive Units multiplied by current average Company buying price, so no loss would be suffered in terms of the current market. Conversely, if prices have fallen since the purchase was made, no windfall could be realized at the company's expense.