amount less then 0 euro of dollar on invoice

Hi All,

I have a question,

we are selling information phone number registrations, like service numbers for call centers.

So for example:

our customer buys toll number with 25c per minute, we pay the customer 15c per minute.
so when his customers call that number for like 5000 minutes in one month, we will pay him 5000 x 0,15c = 750 euro
the monthly fee for the toll number is 30 euro.

So now the problem, i try to make a invoice for our customer but i cant add a artikel below 0 euro

invoice must be:
- montly fee = 35 euro
- call cost    = -750 euro
--------------------------------
total amount =  - 715 euro

its a negative amount that we have to pay the customer BUT it has to be a invoice.

It this possible in one of the new versions of front accounting, or in the future?

We are using 2.2.7

greets Geert de Graef
Shopimport.nl