law: 36. Vacancies not filled up to be carried forward.
  Where in any recruitment year any vacancy under section 33, cannot be filled up due to non-availability of a suitable person with disability or, for any other sufficient reason, such vacancy shall be carried forward in the succeeding recruitment year and if in the succeeding recruitment year also suitable person with disability is not available, it may first be filled by interchange among the three categories and only when there is no person with disability available for the post in that year, the employer shall fill up the vacancy by appointment of a person, other than a person with disability:
  Provided that if the nature of vacancies in an establishment is such that a given category of person cannot be employed, the vacancies may be interchanged among the three categories with the prior approval of the appropriate Government.