AR - Discount on payment

Hello all 

I have ax 2012 

In AR when our customer pay to us some time we give him discount 

so I'm asking how can I enter this discount I try to add it as the picture below but it's don't active to enter any number 

how can I edit this filed ? also Is there any description for this discount ? 

* I know about enter discount from Enter customer payments but I want to enter it in lines as the picture