Need help with AJAX callback function for text field validation in form_alter

Question

I have an existing form with a User field $txn_user for capturing the User by name using the standard autocomplete. So I guess that's a User object, right?

We have many thousands of users and the data entry people would like to -also- be able to enter just the UID (which they have memorised in many cases). So I need a UID field which has an ajax validation to lookup the user by UID and then, update the UserName field.

So in MYMODULE_myform_alter()...

 $form['container'] = array(
  '#prefix' => '<div id="example_form_wrapper">',
    '#suffix' => '</div>',
  );

  $form ['#validate'][] = '_validate_add_userpoints_userid';            

  $form['txn_userid'] = array(
            '#type' => 'textfield',
            '#title' => 'UID',
            '#size' => 4,
            '#maxlength' => 4,
            '#default_value' => isset($txn_user) ? $txn_user->uid : '',
            '#description' => 'The User ID',
            '#required' => TRUE,
            '#weight' => -15,
            '#disabled' => FALSE,
            '#ajax' => array(
              'callback' => 'validate_uid_ajax_callback',
              'wrapper' => 'example_form_wrapper',
                ),  

    );

...and then the field validation callback function

function validate_uid_ajax_callback(&$form, &$form_state) {
    $values = $form_state['values'];
    if( $values['txn_userid'] ) {
        $newuser = user_load($values['txn_userid']);
        if( ! $newuser) {
          form_set_error('txn_userid', 'You have entered an invalid UID. Either leave this blank and select a Username or enter a valid UID');          
    }
    else {  
        $form['txn_user']['#value'] = $newuser->name;       
        $form_state['values']['txn_user'] = $newuser; 
        $form_state['input']['txn_user'] = $newuser->name;          
    }
  }
}

So the ajax spinner goes off and $form gets updated. $form_state does not.

But I can't figure out how do I set the value of the txn_user and txn_user->uid (the fields in the form so they update dynamically)?

Now... if this helps, here is my $form validate function. This -works-. So the only issue is that users (reasonably) expect that when they update the UID field, the User name will update.

function _validate_add_userpoints_userid(&$form, &$form_state) {
    $values = $form_state['values'];
    if( $values['txn_userid'] ) {
        $newuser = user_load($values['txn_userid']);
        if( ! $newuser) {
          form_set_error('txn_userid', 'You have entered an invalid UID. Either leave this blank and select a Username or enter a valid UID');          
    }
    else {  
        $form['txn_user']['#value'] = $newuser->name;       
        $form_state['values']['txn_user'] = $newuser; 
        $form_state['input']['txn_user'] = $newuser->name;          
    }
  }
}

TIA,