There are business cases where one would like to generate an invoice with an external tool and upload it to Blesta. An example would be an IT-Service pvoider that uses Blesta for electronic billing (online services) but uses invoice ninja/.... or stripe invoicing or his bank account built-in invoice tool to generate invoices related to services rendered in physical locations.

Workflow:

Log into blesta as admin
Select user to upload an invoice to
Invoice > Add > Select Upload (new option)
Blesta shows an "invoice number" to use.
Admin generates invoice via third party tool.
Admin uploads the invoice via a dialogue (next step in invoice add after showing an invoice-id)
admin enters invoice amount (resulting in negative account balance) => In this case the invoice should be uploaded to the "cached" invoice dir (invoice caching on = required).
Invoice shows as "open" for both client and admin, and is downloadable.
....
Client pays (with any available payment gateway, or a transaction is manually added by admin via transaction add (this would be the case when using cash or a payment method that does not have a payment gateway yet).
Cron: See that there is enough balance to "fix"/"resolve" the negative balance by applying loose credits. => Invoice gets marked as closed.