Except as otherwise herein provided, the council by resolution shall have full power to make and authorize contracts on behalf of the City with due consideration of short- and long-term cost projections. The city manager shall act as purchasing official, by whom all purchases of supplies for the City shall be made, and who shall approve all vouchers for the payment of the same. All such purchases and sales shall conform to such rules and regulations as the council may from time to time prescribe.