Payment of Variable portion of the salary

I have been appointed as Management Trainee in Agribusiness Unit, SBI, Local Head Office Patna on 1st June, 2015 as a contractual officers. My terms of pay included 70% fixed and 30% variable based on performance with an annual CTC of 7.5 lacs. The performance appraisal was conducted half-yearly and the release of variable portion was to be released after 1 year i.e.31st May, 2016. The eligibility for receiving variable is to secure 90% the minimum qualifying marks with the entitlement of 70% variable. Also as per our terms and conditions it was explicitly mentioned that the renewal of the contract would be based on our achieving the minimum qualifying marks i.e. 90%. I have got extension for 2 years on 1st June, 2016 which makes me eligible for 70% variable. I have my extension letter also with me. Despite getting the extension my variable pay is not released as the top notch officials are reluctant to do so. Inspite of my regular follow up with the HR nothing concrete is happening. Therefore I left with the last option to file a petition against SBI. Can I do so and if yes how to proceed with???