The City Auditor shall not allow the amount set aside for any appropriation to be overdrawn, or the amount appropriated for one item of expense to be drawn upon for any other purpose, or order any voucher paid unless sufficient funds shall actually be in the Treasury to the credit of the fund upon which such voucher is drawn. When any claim is presented to him, he may require evidence that such amount is due, and for this purpose may summon any employee of the City or any other person, and examine him upon oath or affirmation concerning such voucher or claim.