Code for Inventory spreadsheet to add/subtract "on hand" amounts

I've attached an older spreadsheet that I can't quite adapt to the new application although the code is close to what I need. I'm hoping someone can re-work it for me as my knowledge of VBA is very limited.
I would need a new box created to enter quantities.
Once a barcode is scanned into the "barcode box" then a quantity would be entered into the "qty" box. If the barcode is found in Column G then you would be prompted to add or subtract qty entered in the qty box in Column I. If item is not found, a simple "item not found" displayed would be sufficient.


Thanks in advance!!! Please let me know if I need to explain anything further.