Delay in full and final settlement from previous MNC employer

Hi, My concern is regarding full and final settlement amount. Actually, due to some confusion within the Finance team and the HR team of my previous organization, an amount of approximately 5,00,000 was given to me twice however I deserved to receive that only once. I raised that to both the teams about this credited amount and they took a months time to verify and clear this. During this course of time, a heavy tax of approx 80,000 got deducted from my account. Since it was a miss from my organization so they asked me to repay the remaining amount i.e. 5,00,000 - 80,000 = 4,20,000, however I am asked to pay the remaining amount once I claim this tax amount after filling my ITR. This 80,000 is adjusted by my organization in my Form 16 so I need not to worry about that. Problem is I have now left the organization and my FnF is due. Company is saying they will not give my FnF unless I pay them 80,000 in advance. Considering that its there mistake of crediting that amount twice, why should I suffer the delay in getting FnF within standard 45 days TAT. I suggested them to either sign a legal agreement bond with me or I can give them a post dated cheque but they refused for both. Please advise what can I do now as I am in need of money and cant afford to repay them 80,000 unless I get the tax claim. Thanks !!