While no citation of authority is needed for this proposition, we do note that the Federal Courts have held that the burden is on the taxpayer in refund suits not only to show that the Commissioner of Internal Revenue erred in his determination of tax liability but also to establish the correct amount of refund due. See United States v. Anderson, 269 U.S. 422 (1926); Union Pacific Railroad Co., Inc. v. United States, 208 Ct. Cl. 1, 524 F.2d 1343 (1975), cert. denied, 429 U.S. 827 (1976); Fireoved v. United States, 462 F.2d 1281 (3d Cir. 1972). Furthermore, I believe that the Commonwealth, in any event, should be entitled to assert an equitable set-off against the Appellee's claim for refund. While this precise issue may be one of first instance in Commonwealth cases, such a right is recognized in the Federal Government taxpayer suits for refunds under Section 7422 of the Internal Revenue Code of 1954, 26 U.S.C. § 7422.