<form col="8">
    <label name="reference"/>
    <field name="reference" colspan="2"/>
    <newline/>
    <!-- <label name="purchase"/> -->
    <field name="purchase" mode="form" view_ids="purchase_trade.purchase_btb_view_form" colspan="4"/>
    <!-- <label name="sale"/> -->
    <field name="sale" mode="form" view_ids="purchase_trade.sale_btb_view_form" colspan="4"/>
</form>