Filter for not already in external system imported invoices

Hi all,

We wanted to use the "external_id" Field but we can not filter for NULL values (for example [external_id][NULL] does not work). To filter after the invoice "stage" field is also not an option because the user could set the bill stage to "finalized" over the UI by accident and the bill would never be imported.

For now we misuse the "external_credit_memo_id" because this field can be filtered for NULL and NOT NULL Values.

But i think this is not the perfect solution here. Could you add a new function to filter the "external_id" field for NULL values or add a new invoice stage that can only be set via an API Call. If you have any other ideas please enlighten me.

Best reagards