Recently Fired

Was fired 2 moths ago as a Sales Rep. Did good for 3 years then because of what I and co-workers beleived michigan economy couldn't hit quota numbers anymore, perhaps also possibility of age discrimination. Anyways they now want me to pay back some commissions I received in past because some customers cancelled contracts or reduced their services. Some of thee were sold 5 months ago , one was sold 2-3 months ago. I am still inemployed. Since they took away my means of earning an income, I have no way to pay those ones back and since I wasn't there I could not try to save those cancellations, they may have been unhappy with the service, sometimes I could rectify the problems. I know that the service was going down hill and since noone was reassigned to these accounts they may have gotten ignored and fustrated. I have had a few cel phone calls from some customers and I still try to help them. But these 3 particular customers I do not know the exact circumstances. It could also be for financial reasons. All it says was customer name and Two cancelled , one reduced services. If I was working there they could charge back within 90 days and would be deducted in future commissions owed. I received a salry plus commissions. But they fired me. ( I wasn't alone some other past top reps got let go ). I think they are changing their quota structure now and also going with more in experienced reps. Can they do this?