[Albert Moulton]

When a player obviously and grossly misunderstands an amount, I've always seem him able to change his mind as long as there was no action behind him. For example, I've seen a guy throw in the size of the big blind for a "call" when the pot had been raised to his right. At that point, he can pull his big blind back and muck since it was obvious that he didn't see the raise.

I think you should not have turned your hand over until the floor was called. And he should have been able to take back the $100 and decide to call the $500 or fold BEFORE seeing your cards. The fact that he might have shown his aces is irrelevant as long as you haven't shown the straight yet.

Assuming the floor made him call the $400 extra, if he was a "live" player and you felt very confident that you would win more than $400 over the rest of the session with him in the game instead of out of it, then I would give $400 back to him as a gesture of good will. But, $400 is a lot. And if he is "live" then his money will frequently go elsewhere besides back to you. So, I'd probably just keep the full amount and let him leave.

FWIW, I think you shouldn't have unusual chip sizes in your stack for just this reason. The floor was right to make you color down, but you probably should have done that yourself prior to the hand in question.