I have faced the same issue and solved using the below code.
app/code/Vendor/Module/view/adminhtml/layout/sales_order_creditmemo_new.xml
<?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='adjustments'>
<action method='setTemplate'>
<argument name='template' xsi:type='string'>Vendor_Module::order/creditmemo/create/totals/adjustments.phtml</argument>
</action>
</referenceBlock>
</body>
</page>
app/code/Vendor/Module/view/adminhtml/templates/order/creditmemo/create/totals/adjustments.phtml
<?php
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
/** @var \Magento\Framework\View\Helper\SecureHtmlRenderer $secureRenderer */
?>
<?php $_source = $block->getSource() ?>
<?php if ($_source): ?>
<tr>
<td class="label">
<?= $block->escapeHtml($block->getShippingLabel()) ?>
<div id="shipping_amount_adv"></div>
</td>
<td>
<input type="text"
name="creditmemo[shipping_amount]"
value="<?= $block->escapeHtmlAttr($block->getShippingAmount()) ?>"
class="input-text admin__control-text not-negative-amount"
id="shipping_amount" />
</td>
</tr>
<tr>
<td class="label">
<?= $block->escapeHtml(__('Adjustment Refund')) ?>
<div id="adjustment_positive_adv"></div>
</td>
<td>
<input type="text"
name="creditmemo[adjustment_positive]"
value="<?= $block->escapeHtmlAttr($_source->getBaseAdjustmentPositive()) ?>"
class="input-text admin__control-text not-negative-amount"
id="adjustment_positive" />
</td>
</tr>
<tr>
<td class="label">
<?= $block->escapeHtml(__('Adjustment Fee')) ?>
<div id="adjustment_negative_adv"></div>
</td>
<td>
<input type="text"
name="creditmemo[adjustment_negative]"
value="<?= $block->escapeHtmlAttr($_source->getBaseAdjustmentNegative()) ?>"
class="input-text admin__control-text not-negative-amount"
id="adjustment_negative"/>
<?php $scriptString = <<<script
require(['prototype'], function(){
//<![CDATA[
Validation.addAllThese([
[
'not-negative-amount',
'{$block->escapeJs(__('Please enter a positive number in this field.'))}',
function (v) {
if (v.length)
return /^\s*\d+([,.]\d+)*\s*%?\s*$/.test(v);
else
return true;
}
]
]);
if ($('shipping_amount')) {
$('shipping_amount').advaiceContainer = $('shipping_amount_adv');
unblockSubmit('shipping_amount');
}
if ($('adjustment_positive')) {
$('adjustment_positive').advaiceContainer = $('adjustment_positive_adv');
unblockSubmit('adjustment_positive');
}
if ($('adjustment_negative')) {
$('adjustment_negative').advaiceContainer = $('adjustment_negative_adv');
unblockSubmit('adjustment_negative');
}
function unblockSubmit(id) {
$(id).observe('focus', function(event) {
if ($$('button[class="scalable update-button disabled"]').size() > 0) {
enableElements('submit-button');
}
});
$(id).observe('change', function (event) {
enableElements('submit-button');
});
}
//]]>
});
script;
?>
<?= /* @noEscape */ $secureRenderer->renderTag('script', [], $scriptString, false) ?>
</td>
</tr>
<?php endif; ?>