bank-payment/bank_statement_instant_voucher/view/account_bank_statement_line.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="view_banking_bank_statement_form" model="ir.ui.view">
            <field name="name">Add instant voucher button to bank statement line on statement form</field>
            <field name="inherit_id" ref="account_voucher.view_bank_statement_form_voucher" />
            <field name="model">account.bank.statement</field>
            <field name="priority" eval="30"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='line_ids']/tree//field[@name='voucher_id']"
                       position="before">
                    <button name="create_instant_voucher" states="draft"
                            string="Create matching voucher"
                            icon="STOCK_COPY"
                            type="object"/>
                </xpath>
            </field>
        </record>
    </data>
</openerp>