PayTM - Additional cash back offer denied

I had purchased a mobile as there was an exchange offer. I saw the offer mentioned at the time of purchase that I would get Rs.**** additional cash back over exchange value. But after exchanging my old phone I only got my exchange value but not the additional cash back. I contacted the customer care but they told me there was no such offer and asked to send them the offer screenshot. I did not take any screenshot of the offer.
Proof of evidence: I had sent a message to their customer care on the same day of purchase, regarding the additional cash back offer and was asked to wait for ** hours after my old phone was picked up.