Release Letter

I have resigned from my Company on 4/12/2020 and have intimated them that I will not be serving complete Notice period and will not be available after 11/12/2020. I have all the communication through Official Email ID. On 15/12/2020 I have called my Manager and informed him that due to some person issues I will not be able to serve Notice and asked to give buy out option and give me release letter. Since than there is no contact from either HR or Operations and when I asked them about release letter they are saying your case is Absconding and we can not issue release letter. I have almost 25 days salary pending which they are also denying to Pay. In my case earlier with the same company I have resigned and was released within 30 days and they gave me salary and all documents and now when I am asking for clarification no one is responding neither HR or Operations. how should I take this forward to get my release letter.