I note that you are an admitted "newbie" to Access. Inventory is not a trivial subject as has been reported many times in the various forums. Here is a
link to a 26 minute free youtube video that deals with Stock Management.
This doesn't resolve your specific problem, but it does go through the process of design and development of an inventory system. It will show you the things involved generally -- you should get a better understanding of what is involved.
Here is a
link to Allen Browne's article on Inventory which is an often referenced link, especially as it deals with "Quantity on Hand".
Good luck with your project.