The clerk of each county or encumbering officer of the municipality shall keep a record in such form as prescribed by the State Auditor and Inspector in which shall be kept an exact account of each appropriation as made by the county excise board as departmentalized for each department of government or as made by the municipal governing body as authorized by law. The amount and purpose of each purchase order or contract shall be charged against the appropriation as made by the excise board or governing body at the time purchase is made or contract let and the balance in the appropriation account after such charges are deducted shall constitute the unencumbered balance available. No purchase order shall be paid until approved by the officer, board or commission having charge of the office or department for which the appropriation is available and from which such payment is proposed to be made, provided that no indebtedness for any purpose shall be incurred in excess of the appropriation for that purpose and provided that the county and municipal officers referred to herein are made responsible on their official bond for any and all indebtedness incurred by them. Each county or municipal officer in charge of a department or appropriation account shall be allowed to incur indebtedness against all appropriations within his department under the regulations as provided for herein, except when otherwise provided by law; and provided further, that only those municipal officers and employees designated by the governing board shall have authority to obligate the municipality.