The county treasurer shall, under a resolution and the direction of the said county board of supervisors, sell the bonds for cash on the best available terms or exchange them on like terms for the legal indebtedness of the said drainage district evidenced by the outstanding drainage bonds, authorized to be refunded by the resolution authorizing the issue of said refunding bonds, and the proceeds shall be applied and exclusively used for the purpose for which said bonds are issued. In no case shall they be sold or exchanged for a less sum than their face value and all interest accrued. After registration the treasurer shall deliver said refunding bonds to the purchaser thereof and when exchanged for said bonded indebtedness of said district, shall at once cancel a like amount of said drainage bonds.