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. 

process_line = partial(self.env['pos.order.line']._order_line_fields)
        return {
            'name':         ui_order['name'],
            'user_id':      ui_order['user_id'] or False,
            'session_id':   ui_order['pos_session_id'],
            'lines':        [process_line(l) for l in ui_order['lines']] if ui_order['lines'] else False,
            'pos_reference': ui_order['name'],
            'partner_id':   ui_order['partner_id'] or False,
            'date_order':   ui_order['creation_date'],
            'fiscal_position_id': ui_order['fiscal_position_id']
        }
    def _payment_fields(self, ui_paymentline):
        return {
            'amount':       ui_paymentline['amount'] or 0.0,
            'payment_date': ui_paymentline['name'],
            'statement_id': ui_paymentline['statement_id'],
            'payment_name': ui_paymentline.get('note', False),
            'journal':      ui_paymentline['journal_id'],
        }
    
    
    
    
    def _get_valid_session(self, order):