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 the notice period paid as per the employment agreement. However, now they are not providing me with a relieving letter, stating that at least 6 months of service is required to issue the relieving letter, but nothing like that is mentioned on the appointment letter. They have just sent an email stating that your last working day was XXX, and the full and final settlement will be given in the next 45 days.

My next company is asking for a formal relieving letter for onboarding.

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

My ex-employer has refused to give the relieving letter, stating that it requires six months of service. They have informed me that they will only provide an email confirming my last working day and nothing else.