I don't understand.
I would just take the store credit, then go get the bulb I need and use the store credit to pay for it, and then make up the difference or whatever.
I think he just misunderstood, because people are idiots and suck at comprehending shit. So when he said this:
CSFF: well my wife bought 30 inch bulbs and i need 42, can't i just exchange it and pay the difference?"
And you said "No, we can't do it exactly like that" he probably just heard the "we can't do that" part and then disagreed without fully comprehending what you were actually saying. He probably just thought you weren't going to give him what he wants.
When dealing with goddamn retards like that (Just assume EVERYONE is a goddamn retard, by the way. If you do that you'll probably be right a good majority of the time anyway), I would just say "Yeah, we can do that, sir. I just have to give you store credit for this bulb and then you can go pick out the other bulb and pay the difference."
So he'll hear the "yeah, we can do that sir" thing in the beginning and then be slightly less prone to being confrontational. Because he, like pretty much everyone, is stupid. So it's kinda pointless to explain how shit actually works, since he'll probably just misunderstand and be all stupid about it.
And I'd try to get his name from his credit/debit card, so when he says something faggy like "you'll remember me for a long time" I can respond with something like "You're right, Mr. Kalkoski (Or whatever his name is)" while giving him a crazy psycho grin. Just to make him think that I might burn his house down or something. That probably wouldn't work so well for you, since you're a sweet, innocent flower and therefore not very intimidating. But you could find out his address or phone number and then tell his wife that her husband had sexual relations with you. Or something. He deserves it. Fuck people who are douchebags to people that work in retail.