You, Sir, are taking the word of the bank, without going back to the people who actually did the pro-ration, which is the TITLE COMPANY, or the group that agreed to it - YOUR MORTGAGE COMPANY - if the two don't agree, you don't have an accepted HUD 1 and you can't proceed to closing. YOU PAID BOTH OF THEM to come up with those numbers. Your REALTOR is not the person who runs these numbers, just as your Realtor would not have been the person who processed your loan or the person who did the survey on the property or provided you with home owners insurance or the person who did your appraisal. The Realtor coordinates with those individuals to ensure that you get to a smooth closing, but he does not do those functions for you and he is not responsible for being the expert in those fields, that's why you hire experts for those specific functions while the Realtor handles the Real Estate functions.
What makes you so certain that the bank is telling you the truth rather than any of us? All they are telling you is it isn't them and, depending on who you are talking to at the bank, they have no responsibility for it and no reason to even remotely CARE whether you were shorted or you were paid correctly. Furthermore, if the TITLE COMPANY did the figures incorrectly, then they are legally responsible and they and/or their underwriters are insured, however, as I stated earlier, I sit at closings all the time and at every single one of them the buyer is told that if they are closing when the taxes are not yet known, then the pro-ration is based on the last known rate from the previous year. Typically the TITLE company will even make them sign a piece of paper that will state that they acknowledge that they will have to pay the difference if the taxes go up, they cannot go back to the seller for more money. I would suggest that you go back through your closing documents and see if you have that letter in your files.
Realtors have a legal responsibility to put their client's fiscal interests first, before their own. They have a high ethics requirement that they have to follow or they can lose their license. While your Realtor has things that they are responsible for taking care of (and there are a LOT of things that they are responsible for that you as a client never see), they are not responsible for determining the taxes. You PAID the title company to determine the amount that the seller would pay you in taxes, that was part of your closing costs, do not try to blame your Realtor for something that was not part of their responsibility. They are NOT the scapegoat.
You might also want to check with your local government and see if the tax rate now is even the same tax rate that it was in Sept when you closed, since in some cases the amount that is owed may have changed due to schools, fire dept, municipalities and other taxing entities changing their tax structure - this can be due to many things among them: a) laws and regulations that allow some schools and municipal functions to never lose revenue even if the property values decline (ie property values go down and the tax rates automatically go up to compensate) and b) people voting in new taxes for new and improved projects, etc Neither one of these would have been known in Sept and both would have caused your taxes to go up now.
You can state that you don't like your past Realtor and you don't think that your past Realtor treated you correctly, but don't sit there and make a blanket statement that all Realtors are liable for nothing & we don't do "due diligence" or protect our clients & we're only out to get a quick commission & move on. You obviously have no clue what GOOD Realtors do & how hard we work to protect people & take care of them. We don't walk away either, we're still there for them later if they need help.