partner_credit_limit/views/partner_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <!-- make readonly for everyone -->
        <record id="view_credit_readonly_partner_form" model="ir.ui.view">
            <field name="name">res.partner.partner_credit_limit.form</field>
            <field name="model">res.partner</field>
            <field name="inherit_id" ref="account.view_partner_property_form"/>
            <field name="groups_id" eval="[(6, 0, [ref('base.group_sale_salesman')])]"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='credit_limit']" position="attributes">
                    <attribute name="readonly">1</attribute>
                </xpath>
            </field>
        </record>

        <record id="view_credit_readonly_partner_form2" model="ir.ui.view">
            <field name="name">res.partner.partner_credit_limit.form</field>
            <field name="model">res.partner</field>
            <field name="inherit_id" ref="partner_credit_limit.view_credit_readonly_partner_form"/>
            <field name="groups_id" eval="[(6, 0, [ref('base.group_sale_manager')])]"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='credit_limit']" position="attributes">
                    <attribute name="readonly">0</attribute>
                </xpath>
            </field>
        </record>

    </data>
</openerp>