[With regard to any county which is exempt from the provisions of Section 19-2-3, this section shall read as follows:]
In any county having countywide road or bridge bonds or district road or bridge bonds outstanding, and having on hand in the county treasury to the credit of a special fund set aside from the county's share of the gasoline tax for the payment of the principal and interest of such bonds, or to the credit of bond and interest sinking funds, a sufficient amount to pay the principal of and interest on all such countywide road or bridge bonds and district road or bridge bonds outstanding, the board of supervisors shall no longer be required to set aside any part of the county's share of the gasoline tax or the county's share of motor vehicle privilege taxes, to be used in paying the principal and interest of such road or bridge bonds as they mature. In any such case, however, the funds already set aside from the county's share of the gasoline tax or the county's share of the motor vehicle privilege taxes, for the payment of the principal and interest of said bonds, shall be applied to the payment of the principal and interest of said bonds as they mature and shall not be used by the board of supervisors for any other purpose; however, when all of said bonds shall have been paid, any balance remaining in said funds may be transferred by the board of supervisors to the county or district road or bridge maintenance fund.