msopOnModificationSave.php

<?php

if ($modx->event->name != 'msopOnModificationSave') {
    return;
}

$modification = $modx->getOption('modification', $scriptProperties);
if (!$modification) {
    return;
}

/** @var msProduct $product */
$product = $modification->getOne('Product');
if (!$product) {
    return;
}

$sklad = trim($product->getTVValue('sklad'));
if (!in_array($sklad, array('', 'магазин'))) {
    return;
}

$count = 0;
$q = $modx->newQuery('msopModification');
$q->where(array(
    'rid'     => $product->get('id'),
    'type'    => 1,
    'active'  => 1,
    'count:>' => 0,
));
$q->select('SUM(count)');
if ($stmt = $q->prepare()) {
    $count = (int)$modx->getValue($stmt);
}

if (empty($count)) {
    $product->setTVValue('ostatki', $count);
    $product->setTVValue('sklad', 'нет в наличии');
} else {
    $product->setTVValue('ostatki', $count);
}