Hi,

Add this code in py file in your new module to add new field.

 class account_invoice(osv.osv):

    _inherit = "account.invoice"

    _columns = {

            'test_name' : fields.char('Test name', size='120'),

  }
account_invoice()

Add in xml file in new module to add new field.

    <record id="inherit_invoice_form" model="ir.ui.view">
        <field name="name">account.invoice.form.inherit</field>
        <field name="model">account.invoice</field>
        <field name="inherit_id" ref="account.invoice_supplier_form"/>
        <field name="arch" type="xml">
            <field name="account_id" position="after">
                <field name="test_name"/>
            </field>
        </field>
     </record>

Try this

Thanks