if ($oProductGroupVariantProductItem->getSpecialpriceBrutto() > 0) {
    $oSimpleProduct->setSpecialPrice($oProductGroupVariantProductItem->getSpecialpriceBrutto());
} else {
    $oSimpleProduct->setSpecialPrice('');
}

$product->setSpecialPrice(null);
$product->getResource()->saveAttribute($product, 'special_price');