Display - units amount purchased instead of units in stock

I need to display the amount of units purchased instead of the units in stock on products in my cart.. Does anyone know if there is a variable to use for that?? I figured there could be a subtraction of units in stock from total units.. But I am not sure what variable I would use for the total units or if ther is one.. or if there is already a way to display what has been purchased instead of what is left..

Thanks in advance..

You could customize the file:
/admin/includes/modules/category_product_listing.php

And change the code around line 332 from:

                <td class="dataTableContent" align="right"><?php echo $products->fields['products_quantity']; ?></td>

to read:

                <td class="dataTableContent" align="right"><?php echo $products->fields['products_quantity'] . '<br />' . zen_products_lookup($products->fields['products_id'], 'products_ordered'); ?></td>