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
elif Move:
                Move.action_confirm()
                Move.force_assign()
                Move.action_done()
        return True
    def set_pack_operation_lot(self):
        """Set Serial/Lot number in pack operations to mark the pack operation done."""
        StockProductionLot = self.env['stock.production.lot']
        PosPackOperationLot = self.env['pos.pack.operation.lot']
        for order in self:
            for pack_operation in order.picking_id.pack_operation_ids:
                qty = 0
                qty_done = 0
                pack_lots = []
                pos_pack_lots = PosPackOperationLot.search([('order_id', '=',  order.id), ('product_id', '=', pack_operation.product_id.id)])
                pack_lot_names = [pos_pack.lot_name for pos_pack in pos_pack_lots]
                if pack_lot_names:
                    for lot_name in list(set(pack_lot_names)):
                        stock_production_lot = StockProductionLot.search([('name', '=', lot_name), ('product_id', '=', pack_operation.product_id.id)])
                        if stock_production_lot:
                            if stock_production_lot.product_id.tracking == 'lot':
                                qty = pack_lot_names.count(lot_name)
                            else:
                                qty = 1.0
                            qty_done += qty
                            pack_lots.append({'lot_id': stock_production_lot.id, 'qty': qty})
                else:
                    qty_done = pack_operation.product_qty
                pack_operation.write({'pack_lot_ids': map(lambda x: (0, 0, x), pack_lots), 'qty_done': qty_done})
    def add_payment(self, data):