The Director of Finance shall disburse funds only as authorized by resolution or ordinance. No obligation shall be incurred and no expenditure shall be made unless approved by the Director of Finance and certified by him that there is an unencumbered balance of appropriated and available funds. No check for the payment of any claim shall be issued until such claims shall have been approved, in writing, by the head of the department or office for whom the obligation or claim was incurred, and in the case of purchases with the further approval, in writing, of the Director of Public Service.