The free sample gives a coupon. A bought pack gives cashback. Here is what the consumer does in each, start to finish.
At a booth or event. The sample is free, so the reward is a coupon for the next paid purchase, and we track it to redemption.
No cashback here, because the sample is free. The coupon rewards the first paid buy and turns a trial into a tracked customer.
A pack bought in a store, on Amazon or on Zepto. The code on the pack and the bill unlock cashback to UPI.
The same flow works wherever they bought, so every sale lands on one record and we know what was consumed.
The sample is free, so the code gives a coupon for the next paid purchase. Tracked when the coupon is used.
The pack is paid, so the code plus the bill give cashback to UPI, and more off the next time.
The journeys above, clickable end to end.