No bonds (other than refunding bonds issued to refund and in exchange for previously issued outstanding bonds) issued by the city shall be sold for less than par value and accrued interest.
All bonds of the city having been issued and sold in accordance with the terms of this section and having been delivered to the purchasers thereof, shall thereafter be incontestable and all bonds issued to refund and in exchange for outstanding bonds previously issued shall and after said exchange, be incontestable.