Reduce and check stock of Variable Product

Using this code to reduce the stock from inventory by Attribute Value. It works to reduce the stock, but when you have like 100 g in stock and the customer buys 120 g. It doesn't show Out of Stock.

So the following code was created:

<?php

add_filter( 'woocommerce_order_item_quantity', 'filter_order_item_quantity', 10, 3 ); 
function filter_order_item_quantity( $quantity, $order, $item )  
{
    $product   = $item->get_product();
    $stock_quantity = $product->get_stock_quantity();
    $term_name = $product->get_attribute('pa_weight');

    // The 'pa_size' attribute value is "15 grams" And we keep only the numbers
    $quantity_grams = preg_replace('/[^0-9.]+/', '', $term_name);

    // Calculated new quantity
    if( is_numeric ( $quantity_grams ) && $quantity_grams != 0 )
        $quantity *= $quantity_grams;

    if($quantity > $stock_quantity) {
        return $quantity;
    } else {
        echo '<p>Out of Stock</p>';
    } 

}

However, it isn't working...

Is there a way to check the stock from $quantity?

    if($quantity > $stock_quantity) {
        return $quantity;
    } else {
        echo '<p>Out of Stock</p>';
    } 

from Reduce and check stock of Variable Product