How to use the value element to pass values internally from $form to $form_state['value']
The problem is that you'll need to set the type of the correct element to value, and that isn't what you've got at the moment. Notice how you use:

$form['field_devis_lien_fiche']['und'][0]['value']["#default_value"]


To access the field the first time round (which you've confirmed to be working). Logically then, if you want to change the same field to a value type, you need to do so on the same element:

$form['field_devis_lien_fiche']['und'][0]['value'] = array(
  "#type" =&gt; 'value',
  "#value" =&gt; $path,
);