2001 Chevrolet Express Chevy 2500 Computer Problem

I have a Chevy Express 2500, and I just had my mechanic repair the engine. It took me a while to pay my mechanic (13 months) due to my budget/income, but after I finished paying him, he started on the repairs. It took him two weeks to finish the repairs, but for some reason he couldn't get it to start. He called me and asked if I had an alarm or a security chip, to which I answered NO. It worked fine (except the engine) when I took the Chevy to him, I never had a problem with my computer. Later, he calls me up to say that it is fixed and it is working, and he had to call the dealer to have them reset the computer because the computer's security would not let the Chevy turn on. WHAT? He wants to charge me $300.00 for that EXTRA work, something I did not agree to, but to his words "It had to be done" in order for my Chevy to work. What can I do? I feel as if he is just trying to get an extra buck from me. Does this sound legit? Can a computer get corrupted somehow when repairing an engine? Or maybe when the battery died, the computer set off a security feature to not let the Chevy start? I don't want to pay the mechanic for something he might of made up off the top of his head.