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. 

if self.journal_id and self.journal_id.company_id.id != self.company_id.id:
            raise UserError(_("The company of the sale journal is different than the one of point of sale"))
    @api.constrains('company_id', 'journal_ids')
    def _check_company_payment(self):
        if self.env['account.journal'].search_count([('id', 'in', self.journal_ids.ids), ('company_id', '!=', self.company_id.id)]):
            raise UserError(_("The company of a payment method is different than the one of point of sale"))
    @api.constrains('fiscal_position_ids', 'default_fiscal_position_id')
    def _check_default_fiscal_position(self):
        if self.default_fiscal_position_id and self.default_fiscal_position_id not in self.fiscal_position_ids:
            raise UserError(_("The default fiscal position must be included in the available fiscal positions of the point of sale"))
    @api.onchange('iface_print_via_proxy')
    def _onchange_iface_print_via_proxy(self):
        self.iface_print_auto = self.iface_print_via_proxy
    @api.onchange('picking_type_id')
    def _onchange_picking_type_id(self):
        if self.picking_type_id.default_location_src_id.usage == 'internal' and self.picking_type_id.default_location_dest_id.usage == 'customer':
            self.stock_location_id = self.picking_type_id.default_location_src_id.id
    @api.multi
    def name_get(self):
        result = []
        for config in self:
            if (not config.session_ids) or (config.session_ids[0].state == 'closed'):
                result.append((config.id, config.name + ' (' + _('not used') + ')'))
                continue
            result.append((config.id, config.name + ' (' + config.session_ids[0].user_id.name + ')'))
        return result
    @api.model
    def create(self, values):