BBP_UI_CONTROL_BADI (PO implementation)

Hello Gurus,

In the SRM link i need to make the custom field editable in the account assignment section based on the invoice.

if the invoice is NOT created in the ECC, then the custom field should be editable in the SRM.

if the invoice is created in ECC, then the custom field should not be editable.

How to achieve this? which is field to check where the invoice is created or not in the badi BBP_UI_CONTROL_BADI