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. 

return self._create_account_move_line()
    @api.model
    def create_from_ui(self, orders):
        
        submitted_references = [o['data']['name'] for o in orders]
        pos_order = self.search([('pos_reference', 'in', submitted_references)])
        existing_orders = pos_order.read(['pos_reference'])
        existing_references = set([o['pos_reference'] for o in existing_orders])
        orders_to_save = [o for o in orders if o['data']['name'] not in existing_references]
        order_ids = []
        for tmp_order in orders_to_save:
            to_invoice = tmp_order['to_invoice']
            order = tmp_order['data']
            if to_invoice:
                self._match_payment_to_invoice(order)
            pos_order = self._process_order(order)
            order_ids.append(pos_order.id)
            try:
                pos_order.action_pos_order_paid()
            except psycopg2.OperationalError:
                
                raise
            except Exception as e:
                _logger.error('Could not fully process the POS Order: %s', tools.ustr(e))
            if to_invoice:
                pos_order.action_pos_order_invoice()
                pos_order.invoice_id.sudo().action_invoice_open()
        return order_ids
    def test_paid(self):