@Davide is exactly right. With a purchase, you are responding to the offer of a discount the supplier has extended to you. There is no uncertainty. But you still have the choice of using a debit note or adjusted payment as outlined in the Guide: Record early payment discounts on purchases | Manager.
With a sale, there is uncertainty. As the seller, you must carry the full amount of the receivable until your customer decides whether to pay early. Then, you must fully liquidate the receivable while receiving only a portion of it
In either case, you must determine correctly where to apply tax codes to be sure you satisfy your tax authority.