What can I do about money I spent due to a mistake with the credit union?
We cannot give legal advice. The following is not a substitute for the advice of a local attorney. But we hope the information will be useful.
I can understand how you made the mistake. The credit union should be aware that its own mistake initiated this entire unfortunate process, however. They are somewhat suspicious of your "senior moment"/brain slip, or whatever it was due to the large amount you overdrew the account. You need to get them to agree to payments with interest, as if you took out a loan. If you have monthly pension checks coming in, it should not be a problem to make the payments with reasonable interest. $200 a month would have it cleared up in approx. 1.5 years. Not that big a deal, and the credit union makes its money. If you are on fixed income (pension), you cannot realistically pay back the entire amount overnight.
It would be best to get an attorney to represent you, or at least write the bank a letter on attorney's letterhead, proposing a pay-back plan, and perhaps implying a threat of a suit for wrongful prosecution if the bank calls the cops on you due to a problem caused by the bank's own mistake. An attorney would probably write a letter of this sort for you in a deal for limited representation for about $100, give or take. Schedule a meeting with a local attorney and see if they will agree to help you on a limited basis.