Can my boss do this?

I’ve recently put in my two weeks and my boss is trying to inform me that I owe them money for all the training they paid for. ($800). This training was not mandatory however i was never told while signing up for it that I would ever need to pay it back. And I never signed anything stating that I’d need to pay it back. They are threatening to either take the money out of my last check or not give me my vacation hours. What can I do? I don’t have enough money for them to just take $800 dollars.