Unless I'm completely wrong, that's what
Alpha User Points
are for. See the link and search for "AUP" here in the forums.
It's a payment system in the first place (good start for what you need), and you can make the points available to users by creating "products" which aren't really any, but the e.g. $10,000 the user pays for it earns them 10,000 points (or whatever "conversion rate" you set), and with those points they can go shopping.
Sure a coupon can be used to subsidise the points purchase, partially or fully.