Hi Venkat, what I see is that your employer, after hearing about your health issues and that you would not be able to work further, sent you an email asking you to repay the health insurance premium and also to submit your resignation letter officially. It is strange that the company should ask you to pay back the health insurance premium, especially after knowing that you have health issues. This does not reflect well on the company in court.
The email sent to you asking for you to submit your resignation letter officially also serves as evidence that you have discussed with the company regarding your health issues and that you are not able to work any further. So, the company also has the liability to explain to you why it cannot provide you a relieving letter in spite of taking back your insurance coverage and asking you to resign. You can ask the company to provide you the explanation, and if they don't, then you can approach the labor commissioner for further proceedings against the company's actions.