67.732.  Imposition of tax, when — interest and sinking fund, limitations on use. — At the time the county issues the first bonds, the governing body shall impose a sales tax on all retail sales made in the county which are subject to taxation under the provisions of sections 144.010 to 144.525 at the rate approved by the qualified voters.  The ordinance or order imposing the sales tax shall specify that the tax shall expire upon the payment of all principal and interest on the revenue bonds for which the tax was imposed to provide the funds for such payment.  Proceeds from the sales tax shall be used to provide and maintain an interest and sinking fund in an amount adequate to pay the principal of and interest on such bonds and for no other purpose.  Any money remaining in the interest and sinking fund after the payment of all outstanding principal and interest on the bonds may be transferred by the governing body of the county to the county's general revenue fund.