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. 

addons/point_of_sale/models/pos_order.py
"""Create a new payment for the order"""
        args = {
            'amount': data['amount'],
            'date': data.get('payment_date', fields.Date.today()),
            'name': self.name + ': ' + (data.get('payment_name', '') or ''),
            'partner_id': self.env["res.partner"]._find_accounting_partner(self.partner_id).id or False,
        }
        journal_id = data.get('journal', False)
        statement_id = data.get('statement_id', False)
        assert journal_id or statement_id, "No statement_id or journal_id passed to the method!"
        journal = self.env['account.journal'].browse(journal_id)
        
        company_cxt = dict(self.env.context, force_company=journal.company_id.id)
        account_def = self.env['ir.property'].with_context(company_cxt).get('property_account_receivable_id', 'res.partner')
        args['account_id'] = (self.partner_id.property_account_receivable_id.id) or (account_def and account_def.id) or False
        if not args['account_id']:
            if not args['partner_id']:
                msg = _('There is no receivable account defined to make payment.')
            else:
                msg = _('There is no receivable account defined to make payment for the partner: "%s" (id:%d).') % (
                    self.partner_id.name, self.partner_id.id,)
            raise UserError(msg)