About Resignation from Company

I have put my resignation for my higher education on 5th June,2017, after proper discussion with my immediate Boss & General Manager (HOD) and seek to be relieved on or before 22 June by paying settlement for remaining days. (My Company has notice period of 60 days or payment lieu of notice period) At first, my immediate Boss accepted my resignation with remarks "Can not relieve before 15 July", and later my General Manager (HOD) rejected it. After negotiation with my Boss, I was ready to serve up to 15 July. And put my resignation in system again from 5th June to 15 July. Now I agreed their relieving date, but again my General Manger (HOD) rejected it. This time the GM needs the proof of reason i.e. why you are leaving..?? He needs the proof either Appointment letter from would be company or education institute. As my company has resignation thru system, i don't have any written or paper evidences.. I only have mail, got after rejecting from GM. What should i do..?? Is it necessary to provide the proof of other institutions/organisation..? Is it legal..? What is legal action that I can make if necessary..?? Can I leave the organisation on said date i.e. 15 July