Flipkart - Flipkart not taking product back

I had bought a Microsoft Surface Pro on **th Oct ****at Rs *****. This product was not delivered ti me for ** days and then i was told by flipkart to cancel the order and book a new order as they were not able ti track the order. I cancelled the my order and booked the same product at Rs.*****. After * days this product was delivered to me. But serial number on the product and invoice was different. I complained this to flipkart. After * days i got replacement, but again same problem. Serial number on invoice and on product did not match. Now when i asked fliokart to take yhis product back they cancelled my request twice. They are not giving me new invoice nor they are taking this product back. What should i do now.