organization threatening to hold relieving letter

Hello, I joined X organization in August'19 and have left it in January'19. The organization provided relocation since I was not in the same city. Now the concern is, that prior to me joining this organization I served 2 months notice period with my previous organization. During this period and my selection process, I was never for once told about the relocation policy of the organization and how they plan to recover it. It was neither a part of my offer letter. Though it was there in my appointment letter, but it was given to me only after joining the organization. Thereby leaving me with no option to get back to my old job. Now that I have left the organization, they have sent an email claiming 30K or else will not release my relieving letter. Is the organization legally correct, or do I have nay grounds to sue them legally and get back my offer letter.