Can My Employer Legally Extend My Notice Period Beyond What's in My Offer Letter?

My offer letter mentioned that I will be on probation for 3 months, and if I resign (or the company decides to part ways), the notice period will be 15 days. I got a better opportunity during this period and resigned before the completion of my notice period. I had dropped an email and resigned in the HR portal. The HR portal shows the last working day as 15 days from my date of resignation. On speaking with my manager, he is pushing me to stay back for 2 more weeks. Can they legally do that?

I do not wish to stay beyond the official notice period of 15 days, which I am willing to serve.

Please guide me if I stick to my stance of not offering to stay beyond 15 days (as stated officially in the offer letter), can they create any roadblocks in terms of releasing the relieving letter or full and final settlement?

In employment situations, some amount of give and take becomes essential, particularly when you are looking forward to a peaceful parting of ways. You would not like to stay beyond the notice period of 15 days, and you are right in this. Your manager wants you to stay back for another two weeks. You will have to reason with him, and if you attach importance to getting a relieving order as well as a full and final settlement, then you will have to agree with him somewhere in between.