Releiving date

I had resigned on 25 july and after discussion my boss had given me relieving date in writing for 22nd september as i have 3 months notice period and i have to pay 30 days notice period amount which is acceptable to me. But on 1st september HR told me that we can not release you on 22nd September as by boss's boss has asked HR to hold this and communicate to me for completing 3 months notice. Since i have given 24 september as joining in new company, i do not know what to do as this can lead to cancel of my appointment. Kindly suggest.