Unpaid Leaves

Hi, I worked in a MNC for 5 years. I recently fell sick and had taken leave for the same. Since my condition did not improve I had to take all my leaves (annual and sick). I was informed that my leaves got over and will go on unpaid leave to which I agreed since I had no option. Later I was asked to leave job and join back later once I recovered. Now I have received mail from HR asking me to pay for the unpaid leaves that I took or my relieving letter will not be given since the leaves will be given based on pro rata basis. No body ever told me about paying to the co. Now my question is can the Co ask anyone to leave the job provided they r on sick leave with valid reasons. Second should the employee pay to the company for the unpaid leaves taken.