"Tables A and C are separate and need to remain so."
I agree, but have you considered making Tables B and C one table? That approach may make life much simpler for you.
Here's how I set up an inventory log of items removed and added to inventory:
cBalance : ItemsAdded - ItemsRemoved
InvOnHand : Summary, Running total of cBalance, restart when sorted by ItemID
You can create a list time layout that looks and functions much like an accountling ledger for logging and reviewing inventory changes. You can also either create a summary report based on this combined table or base one on table A and use the Sum Function to sum the cBalance field to compute the total on hand for each item listed in Table A.