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
if self.partner_id:
            self.pricelist = self.partner_id.property_product_pricelist.id
    @api.multi
    def write(self, vals):
        res = super(PosOrder, self).write(vals)
        Partner = self.env['res.partner']
        
        if 'partner_id' in vals:
            for order in self:
                partner_id = False
                if order.invoice_id:
                    raise UserError(_("You cannot change the partner of a POS order for which an invoice has already been issued."))
                if vals['partner_id']:
                    partner = Partner.browse(vals['partner_id'])
                    partner_id = Partner._find_accounting_partner(partner).id
                order.statement_ids.write({'partner_id': partner_id})
        return res
    @api.multi
    def unlink(self):
        for pos_order in self.filtered(lambda pos_order: pos_order.state not in ['draft', 'cancel']):
            raise UserError(_('In order to delete a sale, it must be new or cancelled.'))
        return super(PosOrder, self).unlink()
    @api.model
    def create(self, values):