It shall be the duty of the city manager to enforce strictly the provisions of the budget. He shall not approve any order upon the city treasurer for any expenditure unless an appropriation has been made in the budget resolution, nor for any expenditure covered by the budget resolution unless there is a sufficient unexpended balance left after deducting the total past expenditure's and the sum of all outstanding orders and encumbrances. No officer or employee of the city shall place any order or make any purchase except for a purpose and to the amount authorized in the budget resolution. Any obligation incurred by any person in the employ of the city for any purpose not authorized in the budget resolution or for any amount in excess of the amount therein authorized shall be a personal obligation upon the person incurring the expenditure.