You need to have a lawyer look at your "lease extension agreement". If there is a basis for their claim as to another full year, it would be there. Even if there was a year extension, they were under a duty to mitigate after you moved out.
I agree with Mr. Smith. Also, don't assume that the IRS will not make the deficiency an issue down the road. If you were not insolvent at the time and it was not your primary residence, they can treat the deficiency as income, and tax it. But buying the house will not create a problem where there is none, based on what you have told us.