You can either pay the fee--or at least try to work out a settlement you and the landlord can both live with--or you can take legal action. If you take legal action, you can either wait until the collections agency sues you and defend the action, on the grounds you never had a pet--though that runs the risk of having them first report a default or bad debt to the credit rating agencies, at which point you'd have  to take action to remove the false reporting, which is *very* difficult. Or you could pre-emptively take legal action, suing the landlord and seeking a "declartory judgment," or court determination, that  you do not owe the money. No matter what, if the landlord is determined to seek these fees, you will have to spend money and/or time to resolve it; if the fees are comparatively small, the most practical thing would be to pay them, even if you consider them unjust and unwarranted.