Online transaction fraud

Hello, I sold some online assets (voucher) in the exchange of a trade for 6 lakh rupees to a person online for which He transferred the sum from his SBI account to my SBI account (in Uttar Pradesh). After 2 days of successful transaction, bank puts a hold on my bank account amount. I asked the manager, so he told me that an FIR has been lodged in Kerela saying that the transaction done was unauthorised. So I asked what I need to do? Manager said, provide me all the documents and receipt of your trade and we will share it internally then later upon investigation the amount will be released. I did. After 1 year, 2 police officers from Kerela came and met me for my statement regarding this, I cooperated with them and they went back by giving me hope that the investigation is ongoing, we will resolve it soon. Now Yeaterday, 3 Police officers from crime branch came with a Notice u/s Section 91 to produce the stolen amount back by giving consent to SBI that I don’t have any objection. I denied as that will be my double loss, so they said we will arrest you if not paid. Also upon discussion they said that they are unable to trace the main person who did this fraud unauthorized transaction as the IP, Credentials etc are totally spoofed. What can we do in this case on most urgent basis? The police is pressuring me, and also saying if I will not co-operate they will take legal action and will make me accuse. I have tried reaching 1-2 lawyers in my local city but they do not know much about online transaction frauds, please help me with correct advise that appropriate measure we can take at this moment.