You can achieve this by adding the groups attribute in your field definition in the module: create your special group in your module

    <record id ="mygrou_id" model="res.groups">
       <field name="name">My special group</field>
     </record>

in view file add this group in your particular field

    <field name"myfield" groups="my_module.mygrou_id"/>

plz remember to add user for this group

Thanks
Sandeep

As of OpenERP 7.0, you can create field level access rights. You can achieve this by adding the groups attribute in your field definition in the module:

_columns = { 'secret_key': fields.char('Secret Key', groups="base.group_erp_manager,base.group_system") }

In your case, you will have to inherit the target class in the point_of_sale module