Mental harassment by employer and not accepting resignation on mail.

I have given my resignation during probation period since I am getting married and I don't want to work in a toxic company after my marriage to avoid undue mental pressure. I am serving my 1 month notice period currently. They have accepted the resignation verbally and are ready to relieve me but they haven't sent me a formal resignation acceptance mail. My appointment letter says that I can't leave the company for 2 years. This is valid only if I leave the company for better opportunity. I don't have another job in my hand currently but my boss has threatened that if I join any company in future after my marriage, I would have to inform them and pay them my current salary for 2 years. I do not understand why they are doing this and want help regarding this.