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<?php
namespace MyDiscountBlockAdminhtmlSalesOrder;

class Totals extends MagentoFrameworkViewElementTemplate
{
  public function initTotals()
  {
    $parentBlock = $this->getParentBlock();
    $order = $parentBlock->getOrder();
    $source = $parentBlock->getSource();

    $amount = $order->getData("discount_amount"); // custom field

    if (isset($discountHURRY)) {
      $total = new MagentoFrameworkDataObject([
        'code' => 'customdiscount',
        'value' => -$amount,
        'label' => "Custom Discount"
      ]);

      $parentBlock->addTotal($total, 'customdiscount');
    }

    return $this;
  }
}

<?xml version="1.0"?>

<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceBlock name="creditmemo_totals">
            <block class="MyDiscountBlockAdminhtmlSalesOrderTotals"
                   name="customdiscount" />
        </referenceBlock>
    </body>
</page>