If a general obligation bond issue is contemplated, there shall be included in the estimate required by Section 35950 every expense of the district that it is probable will be incurred and become payable before the expiration of one year from the completion of the irrigation works, including the interest on any bonds of the district due and payable prior to that date, for which the money of the district then in the treasury or thereafter to be received from an assessment previously levied is inadequate.