It’s pretty simple, actually! Irrespective of the amount of purchase, the Rs 1,000 are first adjusted. Let’s see some examples-
Purchased for Rs 1,500- Rs 1,000 adjusted first.
Purchased for Rs 4,000- Rs 1,000 adjusted first.
However, if you purchase for less than Rs 1,000 you don’t have to pay anything but the balance is not returned.