Taxes paid on aviation fuel 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 account collected. If in any case the credit, or any part of it, cannot be utilized by the supplier or distributor because of a discontinuance of a business or other valid reason, the amount may be refunded.