// Adiciona pelo ajax
 $output['field_compra_credito']['codvcre'] = array(
            '#type' => 'select',
            '#title' => t('Crédito:'),
            '#options' => $select_cred,
            '#attributes' => array(
                'class' => array('form-control'),
            ),
            '#required' => TRUE,
        );

  $output['field_compra_credito']['submit'] = array(
            '#type' => 'submit',
            '#submit' => array('compra_credito_submit'),
            '#value' => t('Comprar'),
            '#attributes' => array(
                'class' => array('btn-success'),
            ),
        );

    $commands[] = ajax_command_replace('#detalhes_franq', drupal_render($output));
    return array(
        '#type' => 'ajax',
        '#commands' => $commands,
    );