The proposition, when submitted to the electors, shall provide for a sinking fund, to which shall be credited the proceeds of loans sold by or repaid to the county; and such fund, when available, shall be used to pay the interest on said bonds and the principal thereof at maturity; Provided, however, any part of the sinking fund may be reloaned for the purposes specified in sections 10-801 to 10-807, when authorized by the county board.