the term mirror code tax system means, with respect to any possession of the United States, the income tax system of such possession if the income tax liability of the residents of such possession under such system is determined by reference to the income tax laws of the United States as if such possession were the United States. (c) Administrative provisions (1) Definition of deficiency Section 6211(b)(4)(A) of the Internal Revenue Code of 1986 is amended by striking 168(k)(4) and inserting 168(k)(4), and 6431 . (2) Mathematical or clerical error authority Section 6213(g)(2) of such Code is amended— (A) by inserting or section 6431 (relating to economic assistance payments to individuals) before the comma at the end of subparagraph (H), and (B) by striking or 32 in subparagraph (L) and inserting 32, or 6431 . (3) Exemption from offsets So much of any overpayment, credit, refund, or payment as is attributable to the application of section 6431 of the Internal Revenue Code of 1986 shall not be subject to reduction, offset, or levy under section 6331 or subsections (c), (d), (e), or (f) of section 6402 of such Code or under section 3716 or 3720A of title 31, United States Code. (4) Treatment of credit and advance payments For purposes of section 1324 of title 31, United States Code, any credit under section 6431(a) of the Internal Revenue Code of 1986, any credit or refund under section 6431(g) of such Code, and any payment under subsection (b) of this section, shall be treated in the same manner as a refund due from a credit provision referred to in subsection (b)(2) of such section 1324. (d) Appropriations To carry out this section (1) In general Immediately upon the enactment of this Act, the following sums are appropriated, out of any money in the Treasury not otherwise appropriated, for the fiscal year ending September 30, 2020— (A) For an additional amount for Department of the Treasury—Bureau of Fiscal Services—Salaries and Expenses , $78,650,000, to remain available until September 30, 2021. (B) For an