I am a real estate attorney here in the Chicago area, I need to give you a little background on a couple of things to answer your question, as there are a couple of different ways this could have gone:
1. Many, many REO Sellers try to pass on unpaid assessments to the Buyers at closing. The funny thing about this to me is that almost all the contracts state that the Seller will bring assessments current to the date of closing. This is because the law that allows the Selling Bank to pass on the past 6 months of dues (that others have referred to here) is an Illinois specific law, and the addendums to the typical real estate contract that the banks use are written on a national basis. I coach all of my REO buyers to be prepared at closing to make a stand and walk if the Seller to make these the obligation of the Buyer at closing, as it is a clear violation of the contract (realize this is in a situation where the contract states that the dues will be current as of closing). I have found that faced with that option, the Seller's attorney will go back to the bank, explain it to them, and generally we will end up closing a few days later. It is almost impossible to close on the same day, as this has to go through the hierarchy at the asset management office and the bank. As a side note to other posters who will read this - Yes we have tried to work this out prior to closing, sometimes it works, and sometimes it does not. I have actually had Seller's attorney's offices certify in attorney review that all the back assessments will be paid by the Seller, only to show up and closing and have charges on the Buyer's side.
2. In order to close, the Sellers should have had to procure from the association a Paid Assessment Letter, showing how much was due from the Seller to bring the account current as of the date of closing. This is given to the title company so that they can show to the lender who is lending the Buyer money that there are no liens or unpiad assessments on the propety. Often there are disagreements between the Seller Bank and the Association over the amount due. For example, the association may be charging the Seller Bank for unpaid dues from the owner prior to the foreclosure. The Seller Bank will argue that the foreclosure action will remove any responsiblilty on the Seller Bank for upaid assessments from prior to the foreclosure or at least that they are only legally obligated to pay for a certain number of months - usually 6 months prior to the foreclosure (and they are probably right).
So what could have happened? It is possible that the association agreed to drop the amount owed (even if the Seller Bank is not legally obligated to pay all the assessments - i have seen them agree to an amount to avoid having to litigate over it and take months and months to close). In this case a Paid Assessment Letter should have been issued at closing stating the amount that needs to be paid by the Seller to bring the account current, and there should be a matching disbursement from the Seller's side of the Settlement Statement matching that amount. It is possible that due to some oversight at the Association that amount was agreed to by someone, and then not updated in their system, so even with that payment it still shows a balance due. Getting a copy of that paid assessment letter, along with a copy of the Settlement Statement, and showing them to the association should rectify this problem.
It is also possible that the payment has not be received from closing as of yet, and therefore the association is still showing a balance.
In very rare cases when the Seller Bank and the association have not reached agreemeent on the amount due from the Seller, I have seen the Seller place the full amount that the association claims is owed into escrow. This would be reflected on the Settlement Statement as well. The intent would be that they then negotiate with the association over the amount to settle the debt after closing, and the title company still closes knowing that the amount is sitting in escrow to satisfy the debt should there be any problems. If this is the case, then someone needs to talk to the Sellers and the title company to get the money disbursed.
Hope that helps,