hobbzilla

*DISCLAIMER*
I am not a lawyer or a tax professional. This is not legal advice.

From my understanding, for cash based accounting and given that itemize your deductions & you meet all of the IRS guidelines (blah blah blah), you can deduct the amount of property taxes that you pay for in that year.

PROPERTY A.
You paid (in March 2007) $3000 for 2006 property taxes (even if just partial). You sold "A" in June 2007 still owing $7000 for 2006 taxes, as well as 2007 taxes that you incurred while owning the property (Jan '07 - date of sale. I'm guestimating ~$5000 -- based on 2006 total taxes of $10k). So (even though you paid the buyer some of them) you can claim $3000 (2006) + $7000 (2006) + $5000 (2007) for a total of $15k for property A.

Property B.
Purchased in April of 2007. You received $4000 from seller and "forwarded" their $4000 to the tax collector for their 2006 taxes. You cannot claim $4000 (they should).

In addition if and when you pay the entire 2007 taxes (presumably which will be paid in 2008 in 2 installments), you will only be able to claim for 2008 taxes the total amount paid MINUS the amount received from the seller for their portion of Jan07-Apr07, which should carryover from 2007. However, you may be able to claim the entire amount paid for 2007 taxes for your new home for your 2008 tax return given that you treat the amount the seller paid for their pro-rated 2007 taxes as income in 2007. There are only a few reasons why I think you would want to do that though.

That said, depending on where the "taxes paid" are listed on your HUD-1 statement the above may not be true. The real test is when they are paid, paid. And not just paid to escrow (if that is the case here).