Accounting of points for loyalty programms

My company is issuing loyalty cards to customer on certain amount of purchase, once the card is activated the points are accrued every time customer buys in multiple of100. Each point is having a redemption value of 10 paisa. How to account for the points earned by the customer on each swipe. Specifically the liability amount. Whether the liability need to be booked immediately at the time of purchase or it should be done at the time of redemption of the points by customer.