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_session.py
if self.search_count([('state', '!=', 'closed'), ('config_id', '=', self.config_id.id)]) > 1:
            raise ValidationError(_("You cannot create two active sessions related to the same point of sale!"))
    @api.model
    def create(self, values):
        config_id = values.get('config_id') or self.env.context.get('default_config_id')
        if not config_id:
            raise UserError(_("You should assign a Point of Sale to your session."))
        
        
        
        
        pos_config = self.env['pos.config'].browse(config_id)
        ctx = dict(self.env.context, company_id=pos_config.company_id.id)
        if not pos_config.journal_id:
            jid = pos_config.with_context(ctx).default_get(['journal_id'])['journal_id']
            if not jid:
                raise UserError(_("Unable to open the session. You have to assign a sale journal to your point of sale."))
            pos_config.with_context(ctx).sudo().write({'journal_id': jid})
        
        if not pos_config.journal_ids:
            Journal = self.env['account.journal']
            journals = Journal.with_context(ctx).search([('journal_user', '=', True), ('type', '=', 'cash')])
            if not journals:
                journals = Journal.with_context(ctx).search([('type', '=', 'cash')])
                if not journals:
                    journals = Journal.with_context(ctx).search([('journal_user', '=', True)])
            journals.sudo().write({'journal_user': True})
            pos_config.sudo().write({'journal_ids': [(6, 0, journals.ids)]})
        pos_name = self.env['ir.sequence'].with_context(ctx).next_by_code('pos.session')