Excess Full and Final Settlement Paid

Hi, I have been paid an advance salary of Rs 42000 while leaving the company. When I received this amount I assumed that this is my full and Final Settlement instead of an advance salary because I have not received the FnF calculations by then. Had I known that this is an advance salary I would have asked them to take it back as I don't need such advance salary. As per the company they pay this advance salary to help the employee to cover expenses in the notice period (which is written no where) In my case I only served notice period of 1 month (normally its 2 months) after informing everyone (I have proofs of signoff and KT) and also received my relieving letter after all clearance's. Now they want me to give back 17000Rs after all deductions. They have deducted 1 month salary and 18%GST from my FnF for the 1 month notice which I did not serve and since they gave me this advance salary now I have to pay back 17000. I was not informed of such a policy by anyone, not even the HR who was responsible to explain all this to me. Now they have sent a legal notice accusing me of leaving the company without informing anyone (which is a lie which I pointed out to them via email and they admitted that its a standard form which is issued to everyone) but that letter is really malicious and full of lies. what should I do, should I give the money back?