Notice period extension

I have resigned from my company and emailed them the same to them and told them i will be serving my 30 days notice period. My offer letter states that - Either party (Company or Employee) may terminate the employment with one-month notice. The relieving is subject to completion of handing over and knowledge transfer. The final discretion of relieving however lies with the Function head and the Human Resource head based on completion of all formalities.In case of not serving the appropriate notice, short notice period shall be deducted. The Company may withhold relieving/experience letter in case of incomplete handover and knowledge transfer. I am ready to serve my 30 days KT and my immediate manager is agreeing that i can leave in 30 days and he doesn't need KT. But the project head is saying i need to give KT to an intern. I have 6 years of experience and there is no way an intern can learn that work in 30days. They are doing it deliberately it is a small startup type company. I dont care about the money if they deduct it but i need relieving letter and experience letter from them. Please advice what can i do legally to get my relieving letter and experience letter. Hr is not helping as it is a small company hr will do what head will tell them to do.