capo
--- a/CRM/Core/Payment/Smartdebit.php
+++ b/CRM/Core/Payment/Smartdebit.php

    // Update the cached mandate
    $params = [
-      'trxn_id' => $smartDebitParams['reference_number'],
+      'trxn_id' => $smartDebitParams['variable_ddi[reference_number]'],
      'refresh' => TRUE,
--- a/CRM/Smartdebit/Api.php
+++ b/CRM/Smartdebit/Api.php

      case 'PUT':
        $options[CURLOPT_CUSTOMREQUEST] = 'PUT';
-        $url .= '?' . $data;
+        $options[CURLOPT_POSTFIELDS] = $data;