We use internal transactions for invoices and any kind of accounting transactions (in reality, it's not limited to invoice), this is a set of changes to make it easier to record payments between two entities with internal transactions.

• We add a Section Bank Account field directly on the view (
• Reconfigure Causality (Invoices ...) relation fieds to allow allow every accouting transactions portal type, so also Internal Invoice Transaction ..."). This way, when creating the "payment" internal transaction, we connect it to the "invoice" internal transaction and grouping reference is set automatically when posting to general ledger.
• Enable the Create Payment action from (which exists on sales and purchase) on normal invoices.

The rest is small cleanup / fixes described in commit messages.