You'll almost certainly have to retain an attorney to assist you in this matter.
Technically and legally speaking, the bill and the eviction are two separate matters. Whether or not either was ever legally available against your husband depends on a number of facts, such as what the terms of the lease included and, most importantly, the facts relating to the release.
If the eviction/FED case has already progressed through the courts and a judgment has been issued, you may have a couple different avenues to pursue, but likely it will involve filing to vacate the judgment. (This process almost invariably involves an initial review of the facts and case history, then filing a motion and reply with the court and finally going through a hearing in front of a judge.)
Dealing with the bill, then, depends on who's sending it to you. If it's directly from the landlord, you can likely deal with it during the same process as the eviction contest proceeding. If it is instead based on a judgment for back rent that the landlord pursued independently of the eviction, that's a separate matter with a substantially similar process. If, instead, it's a matter for which collection is being attempted by a collection agency, it's a much more amorphous matter, but usually can get negotiated with the agency and the underlying (alleged) creditor directly, especially upon a successful outcome in the eviction proceeding, if available.
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Under the fact you have described your husband should not be responsible for the unpaid rent. Your husband can dispute the reported information with the credit reporting agencies who are providing the information to the potential landlords. If the information is that your husband owes $2,400 in rent then that is not an eviction judgment since landlords cannot recover rent in eviction actions. If your husband was a defendant in an action for rent then you will likely need a lawyer to set aside that judgment. I would recommend getting copies of any court files related to this matter and consulting with an attorney.