Bigbasket.com — cashback reversed from my paytm account via unscrupulous means

Website:

bigbasket.com

I placed an Order worth INR1200 and paid via Paytm. I placed this order only because of the cash back offer with code RAIN with Paytm where on placing an order of at least INR 1200, I would get INR 120 cashback .
I got the cash back immediately.
Next day when the order was delivered, one item worth INR 45 was missing. The delivery boy accepted his mistake.
After a while we see the following message-
Funds Credited due to items billed but not delivered for BBVO[protected].
This was accompanied by the net order value falling below 1200 INR and immediately the cash back of INR 120 was reversed from my paytm account.
All this meant I actually ended up in loss.
I lost my cash back.
I did not received all items I ordered.
I ended up paying quite a bit more as the individual items were cheaper on other sites on that day (I chose Bigbasket only for that cashback).
And now I am being harassed having spent multiple hours talking to customer support and exchanging emails with them with no resolution in sight.
This seems a fraudulent practice from Bigbasket. They first lured me into purchasing from their site using the cash back bait.
Then conveniently forgot to bring one item that made the net order value fall below the minimum requirement.
then reversed the cash back from my paytm account.
The order value falling below the minimum requirement is not my fault.
I would be happy to received that missing item the next day, or even a replacement item.
The fact that they NEVER offered either of their options or any other alternative clearly shows their cheating mind set. They opted for the choice that got them additional INR 120 back into their bank accounts.
The missing item was ENTIRELY their fault, how can they make the customer pay for their mistake.