It shall be the duty of the City Collector and Treasurer at least once every six months during his continuance in office and oftener, if required by Council, to render an account of the taxes, fines, penalties, assessments and other claims in his hands for collection and to read a list of such as he shall not have been able to collect by reason of insolvency, removal or other causes; to which list he shall make affidavit that he used due diligence to collect the same but has been unable to do so, and if the Council shall be satisfied of the correctness of said list and affidavit, it shall allow the City Collector and Treasurer a credit for said claim; he shall pay any money in his hands to the City, upon the order of council and not otherwise.