Upon presentation of any matured bond, or any matured interest coupon of any bond of the district, the treasurer shall pay the same from the bond fund. If funds are not available for the payment of any such matured bonds or interest coupons, both such shall draw interest at the rate of seven percent per annum from the date of presentation for payment until notice is given that funds are available for payment. Whenever the bond fund or sinking fund created for retirement of bonds contains ten thousand dollars ($10,000.00) in excess of the amount necessary to pay all bonds and interest coupons of the district that have matured or that will mature before the time when any part of the next annual assessment, to be levied in the district, will become delinquent, the board of directors may advertise, in the manner herinbefore [hereinbefore] provided for the sale of bonds, for the receipt of sealed proposals for the delivery to the district for redemption of any of its bonds not due. Said advertisement shall state the amount which may be used for the redemption of such bonds. Any such proposals shall be opened by the board in open meeting at the time named in said advertisement, and the offer or offers of such bonds at the lowest rate or rates shall be accepted; provided, that no bonds shall be redeemed at more than par value thereof, except by unanimous vote of the directors. In case two or more proposals are equal, and there is not sufficient money available to accept them all, the lowest numbered bonds shall have the preference. In case not enough bonds are offered for redemption at prices which the board of directors accepts, the board may invest any money available for redemption of bonds in bonds of the United States, or bonds approved for the investment of state school funds, and shall hold the bonds so purchased as part of the bond fund until such time as the board may determine that it is for the best interests of the district that such bonds, or any of them, be sold. In case of the sale of any such bonds the proceeds of the sale shall be deposited in the bond fund. The county treasurer, insofar as the redemption of unmatured bonds are [is] concerned, or the investment of any excess in the bond fund shall act under the order of the board of directors.