Impacted versions: 
9.0x

Steps to reproduce:
1. Go to Accounting, create customer invoice and validate. Check the Invoice-Number (for example: INV/2016/0010)
2.  Go to Point of Sale and select New Session
3. Add any product or just close & confirm the session
4. Close the POS Session (Validate Closing & Post Entries)
5. Create new customer invoice and validate it. Check the Invoice-Number. 

Current behavior:
Gap between the first and the last invoice because of the closed POS-Session. 

Expected behavior:
No Gap between the two invoices like in Version 8. 

I've tested this behaviour on a 8 and a 9 runbot and my own system. There is no gap on 8 but on 9. I think this is not a feature but a bug. I can't find the missing sequence-numbers in any Journals or in the invoice-list. 

The only way to avoid this, is to select another Sales Journal in the POS-Configuration. But then you can't create an invoice from the POS. 

Is it possible to get the "lost" invoice-numbers somewhere? We are not allowed to have gaps in the invoice-numbering here in Germany. 

assert len(self.ids) == 1, "you can open only one session at a time"
        return {
            'type': 'ir.actions.act_url',
            'url':   '/pos/web/',
            'target': 'self',
        }
    @api.multi
    def open_existing_session_cb_close(self):
        assert len(self.ids) == 1, "you can open only one session at a time"
        if self.current_session_id.cash_control:
            self.current_session_id.action_pos_session_closing_control()
        return self.open_session_cb()
    @api.multi
    def open_session_cb(self):