Find available item from an Inventory file, searcg alternative Item if not avail

I all, and thank you for granting me the opportunity to post my first VBA inquiry within this forum.

my starting point is a file with 3 tabs:

tab 1 is a list of items I need to send out (1 column, one item per row, items are not unique)
tab 2 is the status of my Stock (2 columns, first one shows the item code, second one shows the avail quantity)
tab 3 is an "alternative" item list, where I have up to 4 "compatible" items I can search for in case the original item is out of stock

tab 3 structure shown the main items in the first column, and the alternative items on the same rows of the following 4 columns

I know I can INDEX/MATCH and -in case of error on first search- move to the next column and repeat the INDEX/MATCH.
What I do not in VBA is how to change the stock quantity of the items every time I select one from the stock.
What I mean is that, if in tab 1 I have item "x" listed 5 times, and tab 2 shows a row where item "x" has quantity 4, I don't have a clue of how I can assign 4 items and return an error on the 5th one.

Any idea or suggestion?

Thank you and have a great day