I'm not sure...
I think he means I should modify a Email that looks as an official invoice that fullfill the regulations of the german finance office for business invoices and use time_completed as invoice date...
I think this is not possible because this kind of text invoice can be easily modified by the receiver... He just have to click forward and now he can modify the text...
Also the front end invoice overview dont't work anymore.
Also the backend view of invoice don't work anymore.