Winner! 
There is nothing noble about donating money that wasn’t yours. He ain’t Robin Hood.
It’s not his money.
I’m also not so sure they would catch it. My accounting system wouldn’t. I even had a customer pay me $4700 twice. When we called them and told them they told us their accounting system would have never caught it. (and they are a large company) I’m also not convinced they wouldn’t be able to simply credit him.
At any rate, it doesn’t matter.
*
It’s not his money.*
I had $15,000 in tax refunds put in my account in two different years. As soon as they come asking for it, I’ll pay up.
Just kidding. I reported it and it was fixed in a few days.
No it isn’t his money. But the bookstore very likely has no process to cover “hey, you refunded me an extra $100” Giving THEM $100 is likely to result in not getting creditied properly and very likely the extra $100 going into the pocket of whatever clerk is working that day.
Donating it to the university gets it into the hands of where it belongs - without bothering with what is most likely an inadequate POS system run by students who wouldn’t know how to work this even if it had the capability.
I don’t have much advice on the money, except that I’d wait a while to see if they sort it out on their own.
I do have advice on textbooks. I worked in the library when I was in college. I discovered that publishers put out new editions of textbooks all the time so that students will not be able to buy used books. One time I bought a fifth edition of a textbook for $85. I then got the library’s copy of the textbook, which was the second edition. I went through them both, side by side, with a fine tooth comb, and I could not find a single difference (except the cover). I returned the fifth edition, checked out the second edition, and almost never bought another textbook again. I used old editions all the way through college and never had a single problem. It’s worth a try.
Waiting one cycle of the bank to see if the charges work out is quasi-reasonable, but be ready if it doesn’t to go do the Right Thing™. That’s how you’re programmed, bro. And part of what I respect about you.
That’s just nonsense. Every retailer has a system for payments refunds and credits. It may not always work properly every time – but they certainly have a system. It may not even be the store’s error – it could be an error by the credit card processing company.
Donating the money to a third party when you KNOW it’s not yours is a criminal act, pure and simple.
But he did collect $200. It’s just that $100 of it was rightfully his in the first place. 
Doors, I’m with everyone who says, give them one cycle to fix it themselves, and point out the error to the bookstore at that point if they haven’t fixed it by then.
I’d wait for it to resolve, then contact my bank and let them handle it. It is an unauthorized transaction on your account.
This probably won’t comfort anyone, but we recently returned a $50 shower head to home depot. Instead of crediting us, the charged us twice for returning it. Going to the store and talking to the manager didn’t help, we had to call corporate. I’ve been waiting 2 weeks for Home depot to send me a check of MY god damned money.
And yea, I could really, REALLY use my $100. I’m sure it’s all working out somehow in the cosmic wash.
Presumably, you’re short on funds now, while you’re in college, but you intend for that to change after you graduate and get a job. Why not just donate to the alumni fund then, when (hopefully) $100 won’t be such a big deal.
It’s not clear that the bookstore is owned by the college, and even if it is, it probably has to break even on its own, without subsidy from donations.
I agree with those who say wait a week or so and then just call the bank or credit card company. If they tell you to talk to the store, then I would get in touch with a manager , and tell him that you wanted to but honest but that you really can’t tolerate another mistake with your credit card. I can’t imagine they’d let this happen again, but if it does you can always dispute it with your credit card, since you only authorized a charge for $100.
I’ve also had professors be open to students buying older edition texts, and some have suggested this in class. Throughout the course they would even post 2 sets of page numbers, one for each of the last 2 editions. I think its a fair question to email a professor at the start of the course to ask if 2nd-to-last edition would be acceptable, they may say no but I can’t see anyone being annoyed by the question.
Yeah, but buying an old edition doesn’t help when the latest edition is >10 years old already and STILL costs $120. And that’s a book in poor condition.
Damn astrophysics.
Late to the party, but I don’t think it’s an ethical question at all. Airman has already said he has no intention of keeping their money, so the ethical question is settled. It is, however, a question of how to work with a broken system. He’s being hassled by it, but so are they. Imagine being a clerk having to deal with Airman X 1,000.