restrict_invoice_credits/views/account_invoice_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="account_invoice_cred_check" model="ir.ui.view">
            <field name="name">account.invoice.cred.check</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form" />
            <field name="arch" type="xml">
                 <field name="fiscal_position" position="after">
                       <button name="habilitar_check"  states="draft" string="Habilitar Credito"  class="oe_button oe_form_button oe_highlight"
                            type="object" help="Habilitar Credito" groups="account.group_account_user"/>
                       <field name="enable_credit" string='Habilitar Crédito' groups="account.group_account_user" readonly="1"  nolabel='1' states="draft"/>
                </field>
            </field>
        </record>

        <record id="account_invoice_cred_enable" model="ir.ui.view">
            <field name="name">account.invoice.cred.enable</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="factura_venta_golden.account_invoice_cred_cont" />
            <field name="arch" type="xml">
                <field name="contado" position="replace">
                    <field name="contado" attrs="{'readonly': [('state','!=','draft')]}"/>
                </field>
                <field name="credito"  position="replace" >
                    <field name="credito" attrs="{'readonly' : ['|',('state','!=','draft'), ('enable_credit', '=', False )]}"/>
                </field>
            </field>
        </record>

    </data>
</openerp>