How do I find out what my programmer changed?

With my previous site and previous version of ZC (1.3.5?), product available quantity did not decrement until the order was completed. This meant if there was one widget left and User A put it in their cart but spent 20 minutes looking at other items, while ten other users logged on and put that same one item in their carts...and ultimately one of those ten finished checking out...nine users plus User A got error messages because that item was yanked out from under them.

Five-plus years ago, I paid a programmer to resolve this for me - I wanted the available quantity to be decremented immediately when a user clicks Buy Now, with that inventory being restored if the user abandoned the session without buying it - and it worked perfectly.

Thing is, I didn't document the changes he made, and really can't even guess where to look for them so I can make similar modifications to my new site.

Can someone point me in the right direction for this? If I know where to look, I would probably be able to find the custom code he wrote.

Sincere thanks in advance...
Gary

This is how I'd find out about everything that's not the same as a clean set of Zen Cart files:
http://www.zen-cart.com/wiki/index.php/Troubleshoot_-_Diagnosing_Obscure_Issues

Thanks for your reply, but that would be an almost insurmountable challenge...it's safe to say that 80% of the original files had been modified over the five+ years I was using the old app...and I can't think of anything more difficult than looking for a needle in a haystack of needles.

I titled my initial post differently...I'm looking for suggestions re: what ZC files are associated with shopping cart functions. I'm appreciative for the WinMerge suggestion, but I think it'll be much simpler if someone can just come right out and say that "when you click Buy Now, <this file> and <this function> (or whatever) get called".

When you click "buy now", it triggers all kinds of things in hundreds of files.
Here's a list of what happens: http://www.zen-cart.com/index.php?main_page=product_contrib_info&cPath=40_54&products_id=378