Legal Question in Employment Law in California

Send a certified letter back to them saying you do not believe you have been overpaid, particularly because the company (presumably) included the commission as reported wages on your W-2 statement to the government, which you have paid taxes on. Demand that they provide proof or you have nothing further to talk about.

However, just so you know, if they are correct and you were paid money you did not, in fact earn, they have the legal right to pursue an action for the money, if they are inclined to do so. This is true even if it was their mistake. If it comes to that, you will have to decide if the amount of money is worth fighting over. But if the money was included on your W-2, I would insist they file an amended return reflecting the reduction in earnings. This may be enough to dissuade them.