You don't have any good options. You do have a legal right to challenge the bill in court, either by 1) bringing a legal action yourself for a "declaratory judgment" (court determination) as to what you owe, or 2) by waiting to see if they take action against you (e.g. sue you; or a lien) and then defend against it. But if you don't hire a lawyer to help you, you'll be at a huge disadvantage; if you do hire a lawyer, you'll spend more than $500 on the case; and you'll also need some evidence that there was no leak or other reason for the charge (e.g. last tenant leaving something running; defective meter), which may be difficult to get or establish in court. Paying the bill, unfair as it is, is almost certainly your most cost- and time-effective option.