Notice pay recovery - what are the legal consequences if I do not pay the amount?

Hi all, I would like to know if I need to pay if I do not serve the entire notice period.
I was working for a company which has a notice period of 3 months when I received an offer from another company. However, I had to join within 2 months from the offer letter date. Accordingly, I discussed this issue with my reporting manager and the HR. The HR verbally indicated that the 3 month notice period can be waived off and an earlier date of relieving is accepable to the HR if the same is acceptable to my reporting manager. I and my manager mutually agreed for a 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 the resignation and relieving date to HR to complete all the formalities.
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. 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 in the 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.

Failure to pay notice period may forfeit your right to receiving receiving & experience letter. If there is any bond or service agreement then legal course if any else nothing to worry

I have already received a relieving letter from the HR and an experience letter from the Department Head. My query is if I receive a relieving letter and it clearly states that the date is acceptable, then is it not assumed that the notice period has been waived off?

Dear Anamika, having been relieved from the Service of the Company with a specific date (before completion of Notice Period as per the Terms of Appointment) can not be construed as "Shortfall in Notice period is waived, unless the management has communicate the same in writing.You have all the Documents, Please upload.There is no scope for assumption in Legal matters

In that case, the Human Resources (HR) has the moral obligation to inform the employee about the notice pay. Isn't that also a part of the job of the HR? In my case, neither the HR nor the salaries and compensation department informed me about the same.
Moreover, the HR should have intimated me about the notice pay within a couple of months after I left the organization. Is it right on the part of the HR to ask for the notice pay after almost 1.5 years?

After 1.5 years is not at all correct. They should have intimated before giving you the final dues/relieving. You can speak to them, if not agreed you can speak to their higher ups and then proceed.