Extend My Resignation Period day :: Mentally Harassment.

Hello,
I'm Resignation organization on October 15 ,2019 and My last day working is Friday, 13 December 2019.
My Manager approve my Resignation in portal also.
after 45 days HR sending below mail to me.
<< Refer the organisation’s separation policy, you are required to extend the notice till 13th Jan 2020 as this is the site requirement. You will be relieved on the extended last working date only and no incomplete notice will be accepted. >>
But I'm already as per notice period served days. My joining in next company is 16th Dec 2019.
Please suggest and help to raise legal case on this organization.
Last 15 days they are mentally Harassment.

Dear Friend,
Why you are getting worry about it, If you served your notice period as per appointment letter notice period clause then nothing to worry.
Go and join your new company and keep your communications record properly to proceed legally.

Thank You Sir,
Any impact on my pending last 2 month salary and experience letter.
because they are holed my last 2 month salary as per separation policy.

Legal provisions are always sacrosanct. No policy of any organization can run counter to the provisions of any law pertaining to employment. Some of the HR fraternity are happily oblivious of this important principle and in excess of enthusiasm even help design such policies in the interest of the employer. Employment is a simple contract terminable at any time at the option of either party subject to fulfillment of already stipulated conditions in this regard.
When the functional head had already approved the resignation submitted by the employee and upon which the employee had already started serving the notice period more than half-the-way, the employer has no locus standi to extend the same suddenly over and above the stipulated period according to his whims and fancies.
That apart, when the law says that no wage period shall be more than a month and the month's salary should be paid within certain no of days in the succeeding month, how can the policy of separation of an organisation provide for the with holding of the salary of an outgoing employee till the completion of notice period which is more than a month? Even in the case of an employee escaping the provisions of any labor law, such a condition in the contract of employment is certainly against public policy u/s 23 of the Indian Contract Act,1872.