Superior in service confirmed the appointment later denies what to do

I want to know that i was deputed to some where else through department by a examination..before declaration of result Borrowing deptt furnish a certificate of eligibility by which the candidates will be considered...My immediate superior confirm the eligibility and found it correct and signed and swnt to desired end..But at the time of relieving superior says that you are ineligible as you are not fulfilling the eligibility criteria...you cannot relieve I have all documents which he signed is there any legal way to proceed against the competent authority..what will be consequences..and favourism in this legal procedure .. suggest please