Amazon India — Refund is declined

I am posting this because I recently have to face a fraud by the Amazon India. I bought a headphone from Amazon on 11th July 2022 and got delivered on 16th July 2022 after which I applied for a replacement on 22nd July 2022 which I received on 26th July 2022. The price of the headphones was ₹7999. After requesting a refund for the product, the item was picked up by Amazon executive on 1st August 2022 and the pickup boy confirmed that the content of the item was full and original as per the item was received and the serial number was also the same. When the item was reached at Amazon hub they e-mailed me that the item they received is incorrect as the serial number is incorrect and told me to send the right product to them. I told them that I don’t have any product they are talking about because I already sent the product bace to them. So they declined the refund and now are not giving that product back or the refund. Please help me to solve this problem.