Write a stock comparison tool

I need to write a desktop application that takes two excel files as input and compares both files are returns the difference. The first file is the original stock file, that includes item code, barcode, quantity and expiry and the second file is the stocktake file received from the stocktake machine that includes barcode, quantity and expiry. The application needs to look at the new file and finds rows with same barcode and same expiry and merge it and add the quantities. For example, if there exist two rows with barcode 1234 and expiry 01/2018 with first having a quantity of 1 and the second as 2, it needs to merge into a single line with barcode 1234, expiry 01/2018 and quantity 3. The application needs to return the following: Quantity differences based on barcode+expiry Items in original file that didn't exist in the new file Items in the new file that dont exist in the first file Items in the old file with no barcodes