Is my landlord right?

Last month we had a severe thunderstorm and the wind ripped the screen on our window. We called the landlord and told them it was ripped and they came and replaced it. Two weeks later we get a bill for $40. We called and inquired about the bill and they told us there was no way the wind would have caused the screen to rip. What are our rights? Do we have any? We are moving out of town anyway, but I don't think it's fair of them to charge us for the upkeep on their building when we really did not cause the rip at all. If we choose not to clean our carpets when we move out and it exceeds our deposit would they send us a bill or sue us? We are good renters--lived there for over two years with no problems or late rent payments. We paid a very low deposit. Live in Iowa if that matters.