Mark and Carol file a joint return and report itemized deductions $30,000 on their 20x1 Form 1040. Included in the $30,000 of itemized deductions are state income taxes paid of $6,000. In early 20x2, after filing their 20x1 state income tax return, Mike and Carol receive a $600 refund of state income taxes. How, if at all, should the $600 be reported on the couple's 20x2 Form 1040? What is the name of the tax law concept that supports your answer?