If that is the error you are getting I'm not sure if it is this query that is causing it since the column referenced in the where clause in the error is not listed in your query. I can see a couple of issues with the query that you have though. You are not breaking out of the literal for your variables. It should be like this instead:
$query_substractions = "UPDATE products SET Inventory = " . $FinalStock . " WHERE ID = " . $IDItem . "" ;
So long as the query is valid and the values are good it should update the table. You can always print out the query so that you can examine as well. You can do that by adding a die just after setting the query.