Employeer threatning me of legal action for dual employement

I resigned from my organization 40 days back but there is 90 days of notice period. Since I gave my resignation I am continuously requesting early release and offering to buyout the notice period but they were not ready to release me and I have to join somewhere else after a week now. I called my HR again and directly said I will not be able to work further as I will be joining somewhere else. My HR says even if you have stopped working and sent final resignation mail(I have sent) from your side we will not release you from our company system before 90 days and in case you join anywhere else legal action will be taken against you for dual employment. My manager is having no issue releasing me in fact he has already approved my resignation in exit management system but the HR says don't work its fine ,take leave of 45 days and sit idle but we will not release you whatever you do as they need to maintain retention rate. I am from one of the mass recruiting IT company and for them retention rate is a big issue now a days , they are willing to pay salary sitting idle but will not release me.