Impacted versions:
* 11.0

Steps to reproduce:
* Create company A
* Create company B
* Admin user with both companies but currently in company B
* Create Demo user only with company B
* Create fiscal position A to company A
* Create fiscal position B to company B
* Create vendor with fiscal position A with company A and the same vendor with fiscal position B in company B
* Create product and add the vendor created before
* Set like **Create a draft purchase order**
* Logging with Demo user
* Create sale order with one sale order line with route **make to order**
* Validate sale order

Current behavior:
The purchase order created by sale order has the fiscal position **company A**

Expected behavior:
The purchase order must have fiscal position **company B**
addons/purchase/models/purchase.py
name = (self.group_id and (self.group_id.name + ":") or "") + (self.name != "/" and self.name or self.move_dest_id.raw_material_production_id and self.move_dest_id.raw_material_production_id.name or "")
                message = _("This purchase order has been modified from: <a href=# data-oe-model=procurement.order data-oe-id=%d>%s</a>") % (procurement.id, name)
                po.message_post(body=message)
            if po:
                res += [procurement.id]
            
            po_line = False
            for line in po.order_line:
                if line.product_id == procurement.product_id and line.product_uom == procurement.product_id.uom_po_id:
                    procurement_uom_po_qty = procurement.product_uom._compute_quantity(procurement.product_qty, procurement.product_id.uom_po_id)
                    seller = procurement.product_id._select_seller(
                        partner_id=partner,
                        quantity=line.product_qty + procurement_uom_po_qty,
                        date=po.date_order and po.date_order[:10],
                        uom_id=procurement.product_id.uom_po_id)
                    price_unit = self.env['account.tax']._fix_tax_included_price_company(seller.price, line.product_id.supplier_taxes_id, line.taxes_id, self.company_id) if seller else 0.0
                    if price_unit and seller and po.currency_id and seller.currency_id != po.currency_id:
                        price_unit = seller.currency_id.compute(price_unit, po.currency_id)
                    po_line = line.write({
                        'product_qty': line.product_qty + procurement_uom_po_qty,
                        'price_unit': price_unit,
                        'procurement_ids': [(4, procurement.id)]
                    })
                    break
            if not po_line:
                vals = procurement._prepare_purchase_order_line(po, supplier)
                self.env['purchase.order.line'].create(vals)
        return res
    @api.multi
    def open_purchase_order(self):