Flexible Coupon: how to track the value?

We have two kind of coupon: one general and one personal.

The general coupon is a coupon that can be give a discount to some product and it can be used by several people: e.g. I give the code to a group of people that have a 10 Euros/10% discount and they all can use it.

The personal coupon is a sort of product: you buy a coupon (e.g. 50 euros) and you can use it for any kind of our product.

So, the problem is: if a customer have this personal coupon of 50 euros, and buys a product of only 30 Euros, how can he/she spend the other 20 Euros? Is there any solution to automatically update the amount of a specific coupon?

Thanks,
Fabio

No there is no way to update the amount of a coupon automatically.
It can maybe be done by developing a plugin, good php knowledge is required.