Reliving letter not issued by employer - after joining within 15 days another offer has come from an MNC

Hi, I am in deep trouble. Please Help. I have joined a start-up company but after joining within 15 days another offer has come from an MNC. So I have resigned on personal grounds from this company and they relieved me in lieu of notice period paid as per the employment agreement. But now they are not proving me with a relieving letter stating that at least 6-month service is required to issue the relieving letter but nothing like that is mention on the appointment letter. They have just dropped a mail stating your last working day was XXX and full and final will be given in next 45 days.
My next company is asking for Formal reliving letter for on boarding.

As of now you can only produce the copy of the mail indicating the last working day as well seek time to produce final letter after 45 days. You can try to persuade your ex-employer to expedite the relieving letter at least pending finalisation of your full settlement so that your purpose is served.

My ex-employer has refused to give the reliving letter saying reliving letter required six month of service. And not going to give anything except an email stating last working day. Nothing else.