code: protected function preventForbiddenChange(SaleInterface $sale)
      {
          if ($this->persistenceHelper->isChanged($sale, 'customer')) {
              list($old, $new) = $this->persistenceHelper->getChangeSet($sale, 'customer');
              if ($old != $new && (0 < $sale->getOutstandingAccepted() || 0 < $sale->getOutstandingExpired())) {
                  throw new Exception\IllegalOperationException(
                      "Changing the customer while there is pending outstanding is not yet supported."
                  );
              }
          }
      }
docs: Prevent forbidden change(s).
  
  @param SaleInterface $sale
  
  @throws Exception\IllegalOperationException