Calculating quantities using figures rather than number of rows

Hi,

So I send out an Item (lets say 15718) this is in Column A. In Column B I write the quantity I sent out (lets say 12). I then send out the same item at a later date further down the spreadsheet it will appear 15718, and I sent another 4 out lets say.

Then on a separate sheet in the same document I have a returns spreadsheet. On this returns spreadsheet I write in Column A the item number (15718) and in Column B I write how many have been returned (lets say 10).

Now here is the tricky bit... I need to write a formula that states how many have been returned and how many are still out in the field. It wouldnt be such an issue if the quantity was in a list form (so if there was 14 sent out then there would be 14 rows). The issue is the quantity is in Column B as a set number. the other issue is that the Item appears multiple times and the quantity is different both times.

I am trying to get my head round how to do this, and also how to explain this. Is there anyone who could provide me with some help? If you need more info I can try and explain in a clearer way. I have attached a spreadsheet which shows what I am saying in a lot more detail.

If anyone could help it would make my life so much easier, and Im sure its a case of just a couple of formulas. I just dont know them!

Thanks in advance