Employer not allowing buy out and delaying experience letter

I've worked in an IT company for 2.8 years. I resigned from it on 18th January. Company has notice period of 90 days or buy out policy, but they are not allowing to opt by out as the firm has very high attrition rate. I pleaded to HR, my manger but nobody helped. Then I fell ill and submitted my medical to HR with a hope that he might waive off or let me buy out the notice period. I'm in desperate need of experience letter as I've to submit it to my future employer. I've served around 30 days of my notice period in office. Also, for some reason they have kept my salary for feb 2019 on hold, though I was at office till 20th march 2019. Please guide me through this situation.

Time and again, I have been reiterating that existing employees intending to switch their jobs should plan their exit well. Of course, employment can not be a life time affair with a single organization. Everyone is at liberty to choose his/her career in a way most favourable to one's betterment in life. At the same time, he/she should be equally considerate about the inevitable difficulties faced by his/her employer because of his/her exit. One indication towards this positive attitude in an exiting employee is being prepared for the worst of the choices of notice conditions because such an employee knows well about the work culture and exit policy of the organization where he works. Coming to the post, the poster knows well about the high attrition problem faced by his management. If so what prevented him to obtain sufficient joining time from his prospective employer?
Therefore, he can make an appeal to the CEO of his organization explaining his genuine inability to comply with the rendering of the actual notice service as insisted by the organization out of its genuine necessity.