Sales Order to Billing Document copy control
I've added a custom field ZPO in both Sales Order and Billing Document. Now I've implemented the user exit USEREXIT_FILL_VBRK_VBRP so that when a new billing document is created in VF01 it takes the custom field value from the Sales Order. But the problem is this exit is triggered only during billing document creation. I want the value to be changed automatically in billing document when it's changed in Sales Order. Is there any way to do that?(may be user exit or BADI)
Any help will be much appreciated.