Not getting any response on resignation

I work in one of the small IT firms in ahmedabad. I have 1.5 years of agreement as per my appointment letter. I joined the organization in April, 2014. They have taken my one of the original marksheet of engineering and it has been mentioned that it will be returned to me after 1.5 years. It has been 2.5 years that I am working for this company. Still, company has not returned my original marksheet. I got my last appraisal in april, 2016. In my appraisal letter on company letter head, they have written as below : "As per our discussion, currently you don't have any plan to switch the company. So, you are continuing for 1 more year.". This was the appraisal condition. If I need the appraisal than I need to stay for 1 year. Now, I got good offer from one of the MNCs from Ahmedabad. I have resigned from my roles. Now, they are saying that they cannot accept the resignation as I have contract for one year, in fact, that was the appraisal condition originally. But I firmed to be resigned. Then they said they will take the incremented salary difference back. I agreed to return the salary difference for past april to till date as per appraisals condition. Still, They are not responding anything. It was the appraisals condition. As per my appointment letter, it is clearly written about the commitment of 1.5 years. Now, they make me pressurize to work here and saying that I need to complete the project of duration 3-4 months, then they will relive me. They are not providing me this statement in return. They are not hiring another resource. It seems they ignore the matter. I need the reliving letter as well as the original marksheet back. I have 45 days of notice period. In fact,35 days has already passed here without any update and progress from management side. They have relieved many resources in the past with same condition as me. But now, good resources are leaving the company due to bad policies. So, they force the employees with such statements to work there . Please advice What I should do ?