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**
if not self.product_id:
            return
        seller = self.product_id._select_seller(
            partner_id=self.partner_id,
            quantity=self.product_qty,
            date=self.order_id.date_order and self.order_id.date_order[:10],
            uom_id=self.product_uom)
        if seller or not self.date_planned:
            self.date_planned = self._get_date_planned(seller).strftime(DEFAULT_SERVER_DATETIME_FORMAT)
        if not seller:
            return
        price_unit = self.env['account.tax']._fix_tax_included_price_company(seller.price, self.product_id.supplier_taxes_id, self.taxes_id, self.company_id) if seller else 0.0
        if price_unit and seller and self.order_id.currency_id and seller.currency_id != self.order_id.currency_id:
            price_unit = seller.currency_id.compute(price_unit, self.order_id.currency_id)
        if seller and self.product_uom and seller.product_uom != self.product_uom:
            price_unit = seller.product_uom._compute_price(price_unit, self.product_uom)
        self.price_unit = price_unit
    @api.onchange('product_qty')
    def _onchange_product_qty(self):