It is possible to close Invoices (including Recharge Inovice & Credit Note), so that it can't be then further edited.
The 1st step is setting up which group of users can open Invoices - this can be done in a panel PRIVILEGES, tab 'SETTINGS', by marking a checkbox 'Can open invoices'.
The 2nd step is marking a checkbox CLOSE in the Invoice edition window, so that groups of users without the above privilege can't open such invoice (see screenshot on the right).
For all users without the privilege of opening closed Invoice Leon will show an indication as below.
For users who are able to open Invoices, a button 'OPEN' will be active.