What happens when the relieving date from a MNC happens after the date of joining in another MNC

Hi Sir I gave resignation through mail on 01/09/2017. As per company policy I should serve 30 days of notice period. After 4 or 5 days later I informed them that I would leave the company on 15/09/2017 due to some personal reasons and I will be submitting all company properties at the same day. So I requested them to relieve me on 15th. HR was not ready to relieve me on that day ( I did not mention that I have got offer letter from another company). So I left the company on that day submitting all properties while my actual relieving date will be 30/09/2017. So practically I served half of notice period. Then on 17/09/2017 I joined in another company as a fresher. On 21/10/2017 (next month) I got No due Certificate from old company mentioning that I have served notice period fully and reliving date is 30/09/2017. So I did not think twice and confirmed that NDF. But on 23/10/2017 another NDF was sent to me in which number of encashable leave balance is adjusted i.e., reduced. I had to get some amount through leaves but now I have to give them the amount. So I asked HR about this. Then HR told that rest of notice period is adjusted with leave balance. So I agreed it and Confirmed the NDF mail as Yes. Note that the adjustment was not mentioned in NDF. HR told me verbaly. That is it. On 25/10/2017 I got Final Settlement Form through mail in which the amount which I will get is mentioned and the adjustment of leaves was not mentioned again. At first I confirmed. Then I thought I should not confirm and sign that because I served only half of notice period but I am getting One month salary and I thought there may rise dual employment if I get F&F with this relieving date. So I wrote back to them that I dont agree the information furnished in F&F and I asked them to mention my leaving i.e., relieving company earlier along with adjustment of leaves. But HR was not ready to relieve me on the day I left the company. Now there are sending this F&F again and again and I am not sending " I dont agree". If I dont reply within four days it may deemed as confirmation. What is the solution for me getting the salary of 15 days?. Is there any problem if I sign this F&F. I mean is there any chance that they can move against me legally?. I think there is not option in buying out notice period in this company. Hoping reply soon Thank you