Employer Cheated on Final Settlement Amount

Hi Sir, I was working with a company in Gurgaon till Jul 20th 2015, here are some details: Joining Date: Jun 3, 2013 Resignation Date: Jul 1, 2015 Last Working Day: Jul 20, 2015 The employer gives an yearly bonus to employees, which is usually given in 3 parts in 3 months: with May's Salary, Jun's Salary, Jul's Salary. In 2014, I received my bonus in 3 parts - 40%, 40%, 20% with May's Salary, Jun's Salary and Jul's Salary respectively. In 2015, I got 50% of my bonus with Jun's Salary. But as I resigned on Jul 1, now they are saying that I am not eligible for yearly bonus, even they are asking me to payback what they gave me as bonus with June's Salary. However, they have never stated in written or verbal that I will not be eligible for bonus if I resign in the month of Jul. Even there is no any such clause mentioned in the appointment letter. I was eligible for the yearly bonus in June (before resignation), how I suddenly became ineligible after resigning? Second thing, after resigning on Jul 1, I asked for buy out on Jul 10 for the remaining working days. But HR never replied back to me. After few days, when I inquired, they said my last working day would be Jul 20. Now as per my appointment letter clause, if I don't want to serve notice period, then the company would take my 1 month's gross salary. But here, I have served for 20 days. In my settlement letter, they have shown my monthly salary for 20 days only (the days when I was present), additionally they deducted an amount for the remaining 11 days from my 20 days salary. This buyout situation is very confusing. I need advice here. What can I do and is there any possibility of any legal action here? Thanks