Notice Pay recovery BY the Company.

Hi all, I was working for a company which has a notice period of 3 months. I received an offer from another company, However, I had to join within 2 months from the offer letter date. Accordingly, I discussed the matter with my HR manager. The HR manager verbally indicated that the 3 month notice period can be waived off and an earlier date of relieving is accepable provided the earlier relieving date is acceptable to my reporting manager. I and my reportingc manager mutually agreed for an earlier relieving date which would require me to serve a notice period of only 1.5 months as against 3 months. My manager informed the acceptance of resignation and relieving date to the HR deptartment. I had submitted my resignation on 24th April 2016 and was relieved on 10th June 2016. I also received a letter from the Head of HR stating that my relieving date of 10th June 2016 is acceptable to the company. I have also received an experience letter from my department manager on the company's letter head I, now (Dec 2017), receive a mail from the HR stating that since I had not served the complete notice period, i would have to make a payment in lieu of the remaining notice period. Please suggest me if I need to pay the amount and what are the legal consequences if I do not pay the amount.