account_invoice_view.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        <record id="invoice_form_ext" model="ir.ui.view">
		    <field name="name">invoice.form.ext</field>
		    <field name="model">account.invoice</field>
		    <field name="type">form</field>
		    <field name="inherit_id" ref="account.invoice_form"/>
		    <field name="arch" type="xml">
                <data>
                    <xpath expr="/form" position="inside">
						<field name="is_debitnote" invisible="True"/>
					</xpath>
					<xpath expr="/form/sheet/h1/label[@string='Draft Invoice ']" position="replace">
						<label string="Draft Invoice " attrs="{'invisible': ['|','|',('state','not in',('draft',)), ('type','&lt;&gt;','out_invoice'), ('is_debitnote','&lt;&gt;',False)]}"/>
						<label string="Draft Debit Note " attrs="{'invisible': ['|','|',('state','not in',('draft',)), ('type','&lt;&gt;','out_invoice'), ('is_debitnote','&lt;&gt;',True)]}"/>
					</xpath>
                    <xpath expr="/form/sheet/h1/label[@string='Invoice ']" position="replace">
                		<label string="Invoice " attrs="{'invisible': ['|','|',('state','in',('draft','proforma','proforma2')), ('type','&lt;&gt;','out_invoice'), ('is_debitnote','&lt;&gt;',False)]}"/>
                		<label string="Debit Note " attrs="{'invisible': ['|','|',('state','in',('draft','proforma','proforma2')), ('type','&lt;&gt;','out_invoice'), ('is_debitnote','&lt;&gt;',True)]}"/>
					</xpath>				
	                <button name="%(account.action_account_invoice_refund)d" position="after">
	                    <button name="%(action_account_debitnote)d" type='action' string='Debit Note' states='open,proforma2,paid' groups="base.group_user" />
					</button>
				</data>
			</field>
		</record>
		
        <record id="invoice_supplier_form_ext" model="ir.ui.view">
		    <field name="name">invoice.supplier.ext</field>
		    <field name="model">account.invoice</field>
		    <field name="type">form</field>
		    <field name="inherit_id" ref="account.invoice_supplier_form"/>
		    <field name="arch" type="xml">
                <button name="%(account.action_account_invoice_refund)d" position="after">
                    <button name="%(action_account_debitnote)d" type='action' string='Debit Note' states='open,paid' groups="base.group_user" />
				</button>
			</field>
		</record>
	</data>
</openerp>