I'm not a labor lawyer (or any lawyer, nor anyone who knows much about commissions) but what I know from
www.askamanager.org is that they cannot "lower" your pay without notice.
However according to them they are not "lowering" your pay, they are correcting an OVER pay.
It sucks for you, though. And you MIGHT still be taken for a ride, so time to put on your math glasses.
What I'd do:
1) Confirm on your own that they DID over pay you, and by how much. This might mean going to your boss or something to get those reports, because that's important. You can't just be told, "we paid you too much and are taking away MYSTERY dollars for an undetermined period of time." You need to know how much they over paid, and how, exactly, that will impact your future checks.
2) Second, I'd try to get them to recoup the over-payment over a longer period, so your checks aren't 0, even if they are smaller than they were.
3) If they're ass-hats about it, maybe call up the labor department.