The county commission shall annually settle and adjust the accounts of the collector for licenses delivered to him, giving him credit for all blank licenses returned, and charging him for all licenses not returned, according to the statement required to be filed by the person having license; but, when the collector shows that he has exercised due diligence to collect outstanding merchants’ taxes against the merchant and that the same is uncollectible, the county commission, upon a showing of said facts, may allow the collector credit for the amount thereof.