You are smart to have carefully read your written lease with your landlord over the dispute that you are writing about in that its terms and conditions control the obligations owed to you and vice versa in the absence of conflicting state law.If the agreement states that you will be charged for the cost of the plumbing backup ($100.00) if you were negligent, then the landlord has the burden of proving that you caused the plumbing backup that needed fixing and that the cause of the the problem was the result of your negligence.If your position is that you did not cause the problem, then you should stick by your position and no pay the charge.Good luck.