Taxes paid on special fuels represented by accounts found to be worthless, and actually charged off for income tax purposes, may be taken as a credit against subsequent taxes due provided the accounts charged off included the cost of the fuel as well as the taxes due. If the worthless account is subsequently collected, the tax must be remitted on the amount collected. If in any case the credit or any part of it cannot be utilized because of a discontinuance of a business or for other valid reason, the amount may be refunded.