Han owns a farm. He purchases a tractor in 2017 at a cost of $25,000. Because 2017 is a bad year, he does not deduct any depreciation on the tractor in 2017. He sells the tractor in 2021 for $16,000. He takes straight-line depreciation on the tractor of $12,500 for the years 2018 to 2021. The total allowable straight-line depreciation for the tractor for 2017 to 2021 is $15,000. What is Han's gain or loss on the sale of the tractor? Explain.