I don't see how paying cash for fuel would:
a) save money - its the same fuel for the same cost
b) indicate that there is a problem with virgin - if they are have cash flow problems then they don't actually have the cash to pay for the fuel. I would assume that a company in the final death throes would betrying to use credit to overcome any cash flow problems.
Maybe the fuel company has refused to extend credit to virgin, but that's highly unlikely and it would cost them a lot more in paying console operators to collect the cash at each airport.