Really it may be possible in your state as they prepare to file a lawsuit. They probably dropped some or all of the accumulating interest which may make it easier to get a judgment.
I would look at two issues in this case. The first is in the collection case itself - If they can't get the balance straight how can they get a judgment - this is a defense in the debt collection case. The second issue is a violation of the fair debt collection practices act. I believe that the debt buyer is liable for a misrepresentation under that act even though it may have been corrected later.
It is illegal under the state and federal debt collection practices acts to misrepresent the amount or status of the debt. Thus, I agree with Mr. Kuthy that a demand letter for the wrong amount would violate the debt collection practices laws, even if the debt buyer later comes up with the correct amount.
You have the right to demand validation of the debt and until the debt buyer provides validation, it is illegal to continue to dun you for payment. (There are many form letters on the internet that you can look up and use for this purpose.) Here is an article that you may find helpful in understanding your rights regarding the debt validation process: http://www.nolo.com/legal-encyclopedia/debt-collection-defense-requiring-that-the-collector-document-the-debt.html
With debt buyers, you should keep in mind that they often do not have adequate proof of the amount owed. Thus, before you negotiate with the debt buyer, you should consult with an attorney to learn your rights and remedies. Most attorneys will provide you with an initial consultation at no charge. If the attorney finds a violation of the state or federal debt collection practices acts, the attorney may accept your case at little or no cost to you because the debt collection practices acts provide that you are entitled to costs and attorney fees if you win the case.
This is a slam dunk FDCPA violation. Also, be sure to answer any complaint filed in a state court lawsuit regarding the debt as they will have to prove chain of title along with the amount owed. Best of luck!