What would you do if your HR dept did this to you?

It sounds fair, but is not legal, if you were over paid you have to return the money. Because you cashed the checks and by now the money is spent if you do not return the money it is consider theft. Now, I would make them show complete doentation for what was over paid and what is owed to make sure they are not requesting too much money be paid back. Also you can set up a payment plan to cover a couple paychecks so it all isn't taken out at once.