Purchase Order Output - Total Discount Amount

Hi,

our customer wants to see total discount amount of all items in the purchase order on the purchase order output. We tried to make this possible by creating a custom field "total discount amount" and wanted to calculate it in BAdI. We thought that we can use sum of grossamount - netamount of all items but we could not find a BAdI that includes fields in this calculation. Is there another way to do this?

Thank you