invoice_view.xml

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

        <record model="ir.ui.view" id="account_invoice_form_int_number">
            <field name="name">account.invoice.form.int_number</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form"/>
            <field name="arch" type="xml">
                <field name="internal_number" position="attributes">
                    <attribute name="attrs">{'invisible': [('state', '!=', 'draft')]}</attribute>
                    <attribute name="string">Force Number</attribute>
                    <attribute name="help">Force invoice number. Use this field if you don't want to use the default numbering</attribute>
                    <attribute name="invisible">0</attribute>
                </field>
            </field>
        </record>

        <record model="ir.ui.view" id="account_invoice_supplier_form_int_number">
            <field name="name">account.invoice.supplier.form.int_number</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_supplier_form"/>
            <field name="arch" type="xml">
                <field name="internal_number" position="attributes">
                    <attribute name="attrs">{'invisible': [('state', '!=', 'draft')]}</attribute>
                    <attribute name="string">Force Number</attribute>
                    <attribute name="help">Force invoice number. Use this field if you don't want to use the default numbering</attribute>
                    <attribute name="invisible">0</attribute>
                </field>
            </field>
        </record>

</data>
</openerp>