If you are as upset as you sound file a complaint with the better business bureau.
Mistakes happen. They are willing to cover costs they created. Make sure that a note is added on your credit history showing that this was not your fault (if it applies).
The reason for filing a complaint is not the mistake, it is the time for them to restore the money they unlawfully took from you. Sorry, they could legally take X and they took X2 it is an unlawful taking, even if an honest mistake. Had they said oops, it will be in your acccount in a few minutes no one would argue. But 3-10 days, that should include interest for the lost time with your money. Likely only to be a few pennies, but hey, they are collecting that interest. They do not give you 3 days off the interest on your mortgage do they? Electronic transfer of money can be done in seconds. The credit union (by policy) may not admit you have money until the next day however.