It shall be the duty of the board issuing bonds under the provisions of sections 10-501 to 10-509 to negotiate said bonds, and all the proceeds arising from the sale of said bonds shall be paid into the county treasury of said county, and shall be applied solely to the redemption and payment of the unpaid bonds, warrants and indebtedness of said county; Provided, the county board of any county may exchange such bonds at not less than their par value, for the outstanding indebtedness of such county; Provided further, no bonds shall be issued, for the purpose of such exchange, for a less sum than fifty dollars.