It is a bit unclear. It depends on whether you are selling the actual discount (a real world item) or selling the discount coupon (a digital item used within the app). If you can reasonably claim to be selling the real world item then you can use third party payment systems.
If you want to use IAP, consider:
A product that allows users to purchase a service with a limited duration. The content of this in-app purchase can be static. This type of Subscription does not renew automatically. Example: One year subscription to a catalog of archived articles.
...define the 'service' as participation in a virtual club, as an example, where paid membership comes with enhanced features that includes discounts, notifications, ability to make reservations, etc.