Buy app after downloaded with promo code
There is no way to undo a purchase from the end user standpoint, and each user would need to contact apple for a manual adjustment to their account (which might not be something that is even done).

The thing you can control is for that user to set up a second Apple ID to complete a legitimate purchase that qualifies to submit a review. You could also gift the app and be in the clear since that doesn't have the review restrictions that a promotional code redemption does.