invoice_credit/views/account_invoice_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="account_invoice_customer_credit" model="ir.ui.view">
            <field name="name">account.invoice.credit</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form" />
            <field name="arch" type="xml">
                <field name="partner_id" position="after">
                    <field name="contado" attrs="{'readonly': [('state','!=','draft')]}"/>
                </field>
                <field name="contado"  position="after" >
                    <field name="credito" attrs="{'readonly' : ['|',('state','!=','draft')]}"/>
                </field>
                <field name="payment_term" position="replace">
                    <field name="payment_term" class="oe_inline" attrs="{'invisible' : [('credito', '=', False )],'required' : [ ('credito', '=', True )]}"/>
                </field>
            </field>
        </record>

        <record id="account_invoice_supplier_credit" model="ir.ui.view">
            <field name="name">account.invoice.supplier.credit</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_supplier_form" />
            <field name="arch" type="xml">
                <field name="journal_id" position="after">
                    <field name="contado" attrs="{'readonly': [('state','!=','draft')]}"/>
                </field>
                <field name="contado"  position="after" >
                    <field name="credito" attrs="{'readonly' : ['|',('state','!=','draft')]}"/>
                </field>
                <field name="payment_term" position="replace">
                    <field name="payment_term" class="oe_inline" attrs="{'invisible' : [('credito', '=', False )],'required' : [ ('credito', '=', True )]}"/>
                </field>
            </field>
        </record>
    </data>
</openerp>