I was forced to resign due to my medical condition what should I do

I was medically sick, so I took a medical leave. I came back with few medical limitations as reflected in my fitness certificate. I submitted it to Hr Dept and they said that they will get back to me. After 2-3 days they called me and said that I will have to resign or they will fire me on medical grounds( Since I will not be able to perform duties assigned to me earlier). I resigned as my notice period was waved off & was promised a severance pay. They sent me release letter and asked me to pay for my notice period, which I pointed out was wrong. Confused as to how things will unfold now.