Notwithstanding the foregoing, all interest need not be paid annually on an issue of indebtedness provided that either (a) substantially level or declining debt service payments (including all payments of interest) shall be made over the life of such issue of indebtedness, or (b) there shall annually be contributed to a sinking fund created pursuant to this section, the amount necessary to bring the balance thereof, including income earned on contributions, to the accreted value of the obligations to be paid therefrom on the date such contribution is made, less the sum of all required future contributions of principal, in the case of sinking fund obligations, or payments of principal, in the case of serial obligations. When obligations are sold by a county, city, town, village or school district at a discount, the debt incurred for the purposes of any debt limitation contained in this constitution, shall be deemed to include only the amount of money actually received by the county, city, town, village or school district, irrespective of the face amount of the obligations.