A district may borrow money and incur indebtedness, and may issue bonds or other evidences of indebtedness. No indebtedness shall be incurred exceeding the ordinary annual income and revenue of the district without the approval of two-thirds of the voters voting on the proposition to incur such indebtedness except as follows:
(a) A further vote of the voters is not required for any indebtedness heretofore or hereafter incurred within the purposes and not exceeding the available amount of any previously authorized bond issue, and as to such indebtedness the proceeds of any of the bonds unexpended in the treasury of the district, or the par value of any of the bonds which are unsold shall be deemed a part of the ordinary annual income and revenue of the district.
(b) Any district operating a utility under rules requiring applicants for extensions to advance the expenses of such extensions and facilities for serving additional territory may enter into agreements to refund to the applicants in a subsequent year the whole or any part of the expenses so advanced, and the refunds may be paid out of the revenues of subsequent years.