Is it necessary to pay the employer when leaving a company?

Hi! I started working for Wipro BPS from 10th May 2016. On 27th October 2016, while returning from work, I had an accident and broke my left collarbone for which I had to undergo an operation which was performed on 29th. So doctor advised me to take rest and get the checkup done on 10th dec/ After the checkup, he advised me not to join work yet and get another checkup done on 24th Feb. Which I did and finally he told me that after 6 more weeks I can join back. All these time I have updated my TL and also HR. But on 24th HR told me that no further leaves can be provided for me and asked me to either join or resign. So I wrote a resignation letter and emailed it to her on 28th March. She got it approved from the process head and asked me to pay for 2 months (she told me that It would be the basic amount). So I agreed and asked her to tell me the exact amount that needed to be paid but didn't get any response(this was the last conversation we had via email on 18 April 2017). Now, today on 26th September 2017 I got a letter from Wipro stating that I have a due amount of 20k for 2 months notice that needs to be paid and also on the statement details it is written that my last working date was [deleted] but I had the accident on 27-10-16. I was still on probation when I had this accident and as per my agreement letter, it is written that on probation period (which is till 6 months from 10th may) I would have to serve 1 month of notice or a month's salary. So my questions are: 1) What might happen if I do not pay them? Can they take legal action? (I do not have any company assets of any sorts. I was merely just an associate.) 2) Since I was still on probation why are they asking for two months salary?