All moneys received from any such bonds shall be used solely for the purchase, construction, improvement, enlargement, extension or repair of the system, project or combined system for which the bonds were issued, including any engineering, legal and other expenses incident thereto, and to advance the payment of the interest on the bonds during the first three years following the date of the bonds. Any unexpended balance of the proceeds of the sale of any such bonds remaining after the completion of the project for which the bonds were issued shall be paid immediately into the bond and interest redemption fund for such bonds and shall be used only for the payment of the principal of the bonds or, in the alternative, to acquire outstanding bonds of the general issue from which the proceeds were derived by purchase of such bonds at a price (exclusive of accrued interest) not exceeding the face amount thereof. Any bonds so acquired by purchase shall be cancelled and shall not be reissued.