Plumbing and Piping - Over flow issue

I appreciate any and all the help I can get on this situation. There was an "incident" in my apartment in which the water backed up through the wall and caused tremendous damage to my unit and the unit down stairs. My husband had attempted to clean his toliet and in doing so, flushed a very unflushable sponge. When I got home from work that evening, I noticed that the toliet was making more noise than usual and he confessed that he had made the mistake. I called my father who told me to turn that water vavle off and call a plumber in the morning, as it was 8:00 in the evening. At that time the toliet was no where near over flowing, this was just a precautionary measure. The next morning I woke up to a bathroom in about a half an inch of standing water. The toliet had not overflowed, but the water had come through the wall. The downstairs unit had the worst damage as they had noticed the ceiling leaking at 4:00 am and did not call anyone until 7:00. The plumber stated that the valves that were used were cheap and made of plastic? The complex is stating that we are responsible for every last bit of damage that was done. My problem with this is that I feel we took the neccessary steps to prevent the problem, and it seems to me that the problem was with the plumbing which we would have no control over. I almost feel like had we not shut off the valve this whole thing would have never happened. I need to know, are we responsible? Of course, I will take responsibility for the plumber coming out and removing the sponge. After he left, he had replaced a few parts and said it was part of "general maintenance" yet we are also being charged (not much in comparison with the total bill) for these parts. They are listed as 1B1 ballcock and a bowl wax. Any insight anyone has to this situation would be greatly appreciated. Thank you so much.

Was the water leaking from the wall or from the toilet? Did your husband touch any of the valves while he was attempting to clean the toilet? Or move anything?

I believe is came from the wall. My husband shut off the water valve after he flushed the sponge but that was the only valve he touched. The plumber told me that the pressure from the flush caused the leak?? It just sounds a little fishy. The plumber replaced the ballcock, and of couse the bowl wax. I'm thinking that the problem was with the water vavle not working properly or the ballcock itself just leaking. From what I know, had the ballcock leaked after the water was shut off it would have only leaked water that was in the toliet, which would not have caused as much damage as there was. Am I way off base here? I appreciate you taking the time to help me out=)