Notwithstanding any tax levy limitations contained in any other law or city home rule charter, when any city or village of this state elects to accept the provisions of sections 68-601 to 68-631 relating to old age and survivors insurance and enters into a written agreement with the state agency as provided in such sections, the city or village shall levy a tax, in addition to all other taxes, in order to defray the cost of such city or village in meeting the obligations arising by reason of such written agreement, and the revenue raised by such special levy shall be used for no other purpose.