creating null payment with discount

Hi
In order to solve my promp discount probleme I tried the following :
- Go to customer payment
- allocate some payment (of the amount of the discount I want to offer)
- set a discount equal to the allocated payment
-submit

The idea is to "allocate| a discount without paying anythin . The problem is the systeme refuse to do it because the final payment is null
(which is what I want, I just want to allocate a discount ).

I  it should be fairly easy to change the null payment validation if a discount amount is present. However I was wondering if it safe to do so or if the systeme is expecting somewhere else payment to be not null ?

/Elax