Inventory tracking - vlookup with subtraction from running total

Hello,

I hope I can explain this clearly enough to get a little help. I've tried looking for an answer by haven't found anything that quite works.

The Problem:

We receive orders from our customers that have the same SKU on multiple PO's (purchase orders). We need to move the inventory from its current location to the DOCK from 4 other locations. The inventory does not go directly to the DOCK. It usually has to pass through 2 or more locations to get processed. so for example SKU0001 starts in LOC2, moves to LOC3 and then gets moved to the DOCK.

What I need:

I have 3 different orders (PO's) that need SKU0001:
PO9999 need 25 units of SKU0001
PO8888 need 10 units of SKU0001
PO7777 need 50 units of SKU0001

I have a grand total of 100 units of SKU0001 in stock, plenty to fulfill the order. The location of the inventory is as follows:
30 units of SKU0001 in LOC1
10 units of SKU0001 in LOC2
20 units of SKU0001 in LOC3
15 units of SKU0001 in LOC4
25 units of SKU0001 in DOCK

I need to be able to determine how many units of SKU0001 have been moved to the DOCK, how many units are in either LOC1, LOC2, LOC3 and LOC4 and how many units are missing to fulfill the order. As I mentioned SKU0001 will appear on the orders tab multiple times as it will be on multiple PO's, so I also need to be able to subtract the number that have been moved to the DOCK from the running total so the next instance of SKU0001 on the list has an accurate location count for the 5 locations listed above.

Right now we are having to manually enter the numbers based on the locations of the inventory, with a few SKU it's not a problem, bu we usually have about 500 SKU and with multiple updates required through the day, it can be very time consuming to update the data.

Any help would be greatly appreciated.