credit_cash_check/views/purchase_order.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
  <data>
    <record id="purchase_order_form_invoice_credit" model="ir.ui.view">
      <field name="name">purchase_order_form_invoice_credit</field>
      <field name="model">purchase.order</field>
      <field name="inherit_id" ref="purchase.purchase_order_form"/>
      <field name="arch" type="xml">

        <field name="payment_term_id" position="replace"/>
        <field name="picking_type_id" position="after">
          <label for="contado" string="Forma de pago"/>
          <div>
            <label for="contado" string="Contado"/>
            <field name="contado" attrs="{'readonly': [('state','!=','draft')]}"/>

            <label for="credito" string="Crédito"/>
            <field name="credito" attrs="{'readonly' : ['|',('state','!=','draft')]}"/>
            <field name="payment_term_id" options="{'no_create': True}" attrs="{'invisible' : [('credito', '=', False )],'readonly': [('state','!=','draft')]}"/>

          </div>
        </field>

        </field>
    </record>

    </data>
</openerp>