How do I make discount voucher on order amount useable multiple times?

[sm3004]

When refunding a product from an order, there is the option to create a credit slip or a discount voucher. When creating a discount voucher, it is automatically generated and emailed to the user. The voucher generated is automatically set to "Discount on order (amount)" and if it was for $80, then the "Minimum amount" value is set to $80 as well.

Now my question: How do I make it so the customer can use the voucher and spend less than the $80 and the voucher automatically subtracts whatever they use. So in my case, the customer uses the voucher and spends $60, so now they should still have a $20 voucher left.

At the moment, Prestashop just disregards the voucher as used and the $20 are lost. Doesn't matter if I set "Minimum amount" to 0 or anything else, it still won't keep the $20.

Any ideas how to make this possible so I don't have to check every discount?