My Landlord Is Trying To Bill Me For Their Negligence, What Should Be My Next Step?

It rained really hard here, which doesn't happen very often. My apartment faces an interior patio, and has a wooden fence around it, with 2x4s at the bottom, which act as a dam. The drainage for the building happens much more over my patio, than that for other units. This caused a small amount of flooding of my unit. I broke out some of the bottom of the boards on the fence and wedged them open, which allowed water to drain out to the rest of the courtyard. Once the patio was flooding, some of the bagged potting soil I had stored there washed over the drain. The manager said he is considering that the reason for the flooding. They're now trying to charge me $200, saying I asked their workers to cut a hole in the fence, which as their agents, I said they should do whatever they felt was necessary. They have sent me that invoice.

I had minor flooding like this once before. When I told them about it, they didn't even bother to come look. I just had to clean up everything myself.