Maintianing Inventory Quantities

I used the Northwind database and modified it to suit my needs. This is basically just maintaining inventory, using the "Orders" form and "Orders Subform" to create records in the "Orders" and "Order Details" table which deduct quantities from the "products" table.

Our employees are allowed to use the "Orders" form to issue items from inventory, and this is the method I used for updating quantities in the "Products" table (method 1):

http://support.microsoft.com/kb/252813

Here's the problem: It is entirely possible that a transaction from inventory might be cancelled, and there is no way to replace the inventory levels in the "Products" table, other than doing it manually.

I would prefer a method of maintaining inventory levels in the "Products" table based on current records in the "Order Details" table, whereas, I could merely open the "Order Details" table, delete the canceled transaction (record), and the quantities in the "Products" table would be automatically adjusted.

It doesn't seem like this should be too difficult, I just need a point in the right direction on how it should be done.

Any help will be appreciated.

Thanks,
Mark A.