You can filter out deleted invoices. Sales -> Invoices -> Show invoices with status: deleted, when you choose that filter and click on Show then you can see the invoices that have been deleted.
You are not able to open them, but you can see the Invoice number and date and open the preview or PDF printout.
You cannot restore an already deleted invoice yourself. You can re-enter the invoice according to the preview or PDF printout (inventory transactions occur at the moment of confirmation).
Another option is to order the recovery of a deleted invoice from Erply development (paid service, contact customer support at firstname.lastname@example.org). In this case, however, it must be kept in mind that the restored document remains in an unconfirmed status, and inventory transactions are created at the moment of its confirmation.
Customer or customer group
Sales document or purchase document (restored in unconfirmed status, inventory transactions occur at the moment of confirmation)
Product group or product series
Work order or project
Customised printout template (ActualReports template)
In case of deletion or overwriting, it is not possible to restore
Inventory registration act and write-off
Report made by the Report Generator
Inventory transfer or inventory transfer order
Cannot be adjusted: warehouse price on sales documents or on inventory transfer (FIFO method, more information here).
Please note that restoring deleted data is a paid service.