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 a legal indebtedness of the said district evidenced by bonds, warrants, or judgments outstanding at the date of the passage of the resolution authorizing the issue thereof, 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 at the date of sale or exchange. After registration the treasurer shall deliver said bonds to the purchaser thereof and when exchanged for indebtedness of said district shall at once cancel all warrants or bonds or secure proper credits therefor on judgments.