The governing authorities of any municipality authorizing revenue bonds pursuant to the authority granted in Sections 21-27-23 and 21-27-51, may make provisions for any of such revenue bonds to be called for payment at any interest payment date before maturity, provided the municipality shall have on hand in its bond and interest fund sufficient moneys, not otherwise appropriated or pledged, in excess of the interest and principal requirements within the next two succeeding calendar, operating or fiscal years.