What ws the reason cited by the authorities while denying to pay you the pay as per the present post that you are entitled?
Is there any substantial reasons or any law quoted by them for denying you the benefit of the higher post/promotion?
If there is no reason mentioned or if you feel that this act is totally unjustified and illegal, you may first make a representation in writing to the competent authority through proper channel venting out your grievances and seek its intervention and relief.
If you dont get any proper response or the desire relief or you still feel humiliated and aggrieved by the acts of the competent authority, you may approach the state administrative tribunal with an OA mentioning all your grievance and plead that you have been meted out with injustice in this regard and seek its direction to the authorities concerned to provide you the releif as per law and a remedy for this.
You may contact a lawyer in the local who practices in service law for further advises and suggestions to move on.