Taxpayer files a 20X0 return on April 15,20X1 showing a tax liability of $500. On April 1,20X4 he files an amended return showing tax liability of $700 and he pays additional $200. In 20X5, he files a claim for refund of $300 alleging that correct tax liability for 20x) was, in fact, $400. What is the maximum refund which the taxpayer may obtain.