I did get a response, and BeyondCompare was recommended. It's an excellent product. I compared the two files, and I found no suspicious change (as I expected).
I think what we need is a "scammer alert" program -- something that flashes on the screen and rings a bell saying, "SCAM ALERT! The person who is controlling your machine is a scammer. His IP address has already been noted and sent to the FBI and their contacts in India. Please do not give him your credit card details."
Set this program to run on a specific keystroke (say, Ctrl-Alt-Q), and press that keystroke just before you give him your details.
Now, does anyone know how to write this on a Windows computer?