Hello Antonio
You can use the Form Template Selection to assign the templates based on the Bill to Party and Seller/Invoicing Unit.Kindly follow the below mentioned steps to maintain the Form Template Selection rules.
1.Go to Application and User Manangement Workcenter
2. Go to Form Template Selection View
3. Search for Customer Invoice.
4. It will list all the templates maintained for Customer Invoice.
5. You can edit the individual templates and maintain the rules.
You can refer to the below screenshots for better understanding.
Kindly revert back in case of further queries/clarifications.
Thanks
Parveen Kumar