Whenever there are included, within the gross receipts used to determine the license fee of a business under this chapter, amounts which reflect sales for which credit is allowed, and such amounts prove uncollectible in a subsequent year, those amounts may be excluded from gross receipts in the year they prove to be uncollectible; provided, however, if the whole or a portion of such amounts excluded uncollectible are subsequently collected, they shall be included in the amount of gross receipts for the period when they are recovered. (Ord. 5017-NS § I (part), 1977)