pagare_imperio/views/account_invoice_view.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
	<data>
        <record model="ir.ui.view" id="account_invoice_button">
            <field name="name">account.invoice.button</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form"/>
            <field name="arch" type="xml">
				<xpath expr="//button[@name='invoice_print']" position="replace">
					<div class="pagare_button_box" attrs="{'invisible': [('state','not in',['open','paid'])]}"></div>
				</xpath>
				<xpath expr="//button[@name='action_invoice_sent']" position="attributes">
					<attribute name="invisible">1</attribute>
				</xpath>
            </field>
        </record>
	</data>
</openerp>