account_voucher_comision/views/account_voucher_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>

        <!-- FORM -->
        <record id="view_voucher_form_comision" model="ir.ui.view">
            <field name="name">account.voucher.form.comision</field>
            <field name="model">account.voucher</field>
            <field name="inherit_id" ref="account_voucher.view_vendor_receipt_form"/>
            <field name="arch" type="xml">

                <field name="amount" position="after">
                    <field name="comision" readonly="1"/>
                </field>

            </field>
        </record>

        <!-- TREE -->
        <record id="view_voucher_tree_comision" model="ir.ui.view">
            <field name="name">account.voucher.tree.comision</field>
            <field name="model">account.voucher</field>
            <field name="inherit_id" ref="account_voucher.view_voucher_tree"/>
            <field name="arch" type="xml">

                <field name="amount" position="after">
                    <field name="comision" sum="Total Comisión"/>
                </field>

            </field>
        </record>

    </data>
</openerp>