Wow isn t your employer a bad un . Tsk tsk just what is AMERICA coming down to and what would the father founders have to say about THAT, hey?
If they have overpaid you then they (unfortunately) have the right to ask for this money back. So you are not really working for free, you have already been paid for the 30 hours work.
They cannot do both. Either you work for free for the 30 hours OR they can garnish your wages, but NOT both.
They re not making you work for free . They ve already paid you in advance for this time! You owe it to them.
Just GIVE THE MONEY BACK then, you pillock.