1) edit the WADAInsertRecordID form element to capture the Inventory_ID query string variable that is passed over:
<input name="WADAInsertRecordID" type="hidden" id="WADAInsertRecordID" value="<?php echo(isset($_GET['Inventory_ID'])?$_GET['Inventory_ID']:""); ?>" />
2) In the update record behavior, do not bind the Inventory_ID column to a value. the primary key column should be given a value, it will get it's value from the database.