(petitioner inherited partial interest in home owned by parent and began using home as principal residence in 1999; in 2001, petitioner owned second tract on which home fully constructed in 2008 at which time petitioner moved into home; first-time homebuyer credit claimed on home and IRS denied credit on basis that petitioner had ownership interest in residence as of 2007; on petitioner's 2010 and 2011 returns, petitioner repaid $500 of credit on each return and petitioner argued entitlement to credit because IRS accepted repayment amounts; court noted that filing of return establishes petitioner's tax claim position but does not create contract between IRS and taxpayer; court denied offset of $1,000 against outstanding tax liability due to credit denial, but suggested that petitioner file for overpayment refund)