(a) The net bonded indebtedness incurred for all public purposes shall not at any time exceed ten percent of the assessed value of all the real and personal property in the City; provided that in computing such net bonded indebtedness, there shall be excluded all money borrowed which, by law, does not constitute an indebtedness of the City within any constitutional or statutory debt limitation or which is permitted by law to be in excess thereof. The resources of the sinking fund pledged for the retirement of any outstanding bonds shall also be deducted from the amount of the bonded indebtedness.
(b) No bonds shall be sold to obtain funds for any purpose other than for which they were specifically authorized, and, if such bonds are not sold within three years after authorization, such authorization shall be null and void.