X-Cart forums - show remaining amount on coupon

I want to use coupon to allow customer to buy prepaid accounts.
For example he pays 1000 and receives a 1000 worth coupon which he can later use at his own discression to make any amount or purchases.

I don't know if there is some mode that can do exactly this...
But for now using coupons is the quick fix I came up with.

The thing that is missing from this whole scenario is that XC doesn't show remaining amount on the coupon after it's been partially used.
Is there a defined variable to show the remaining amount on the coupon?

To illustrate
coupon 1000 usd
customer makes purchase 100 usd
I want to see 900 usd left on coupon