Unable to get relieving letter

I was working with a company for 6 months and had to leave it because of personal reasons. I spoke with the company manager for a buy out but they were not willing to go ahead with that option and told me that I must serve the notice period. I wrote an email of resignation but they did not accept it saying I need to come back after my personal issues are sorted and still serve the notice period. I did not respond to that email and left the company. My next employer was okay with me not having the relieving letter. Now I got an offer from the 3rd company and left my previous employer and 2 weeks before my joining I got an email from the new company that they are putting my joining on hold because I do not have the relieving letter from my first company. I spoke with the HR and the general manager at the old company requesting them to provide me with the relieving letter, but they are refusing me the same. As per their appointment policy, I won't be getting the relieving letter as well as the last months pay. I want to know is there any chance I can get a relieving letter from the employer or a way to convince the new company to go ahead without the relieving letter? Can I request for a buy out now? My job is at stake and I have time only till 9th September, 2019. Please help me.