20 C.F.R. § 345.118 – Adjustments.

(a) In general. If more or less than the correct amount of an employer's contribution is paid with respect to any compensation, proper adjustments with respect to the contributions shall be made, without interest, in subsequent contribution payments by the same employer, as provided for in this section.
(b) Compensation adjustment. A compensation adjustment is the amount of any adjustment reported by an employer on Form BA-4. See part 209 of this chapter.
(c) Adjustment of contributions.
1. (1) All adjustments of contributions based on compensation adjustments shall be accounted for by the employer on the contribution report for the same quarter in which the Form BA-4 reflecting the compensation adjustments is filed with the Board.
2. (2) If less than the correct amount of contributions is paid for any previous calendar quarter or calendar year because of an error that does not constitute a compensation adjustment as defined in paragraph (b) of this section, the employer shall adjust the error by—
  1. (i) Reporting the additional contribution on the next report filed after discovery of the error; and
  2. (ii) Paying the amount thereof to the Board at the time such report is filed.
3. (3) If more than the correct amount of contributions is paid for any previous calendar quarter or calendar year because of an error that does not constitute a compensation adjustment as defined in paragraph (b) of this section, the employer shall adjust the error by applying the excess payment as a credit against the contribution due on the next report filed after discovery of the error. However, if the overpayment cannot be adjusted because the employer is no longer required to file a report or because the overpayment to be adjusted exceeds the amount of contribution due on the employer's next report, the employer may file for a refund of the amount which cannot be adjusted as provided for in this section. If the overpayment is the result of an incorrect contribution rate as determined by the Board, the employer may file for a refund of the amount of overpayment or may take an adjustment as provided for in this section.
(d) Limitations on adjustments. No overpayment shall be adjusted under this section after the expiration of three years from the time the contribution report was required to be filed, or two years from the time the contribution was paid, whichever of such periods expires the later, or if no contribution report was filed, two years from the time the contribution was paid. Any underpayment not adjusted within the time limits as set forth in paragraph (c) of this section shall be adjusted on the employer's next contribution report or reported immediately on a supplemental return. Interest shall accrue on such underpayment as provided for in § 345.122 of this part from the time the adjustment should have been made under paragraph (c) of this section to date of payment. However, no underpayment shall be adjusted under this section after the receipt from the Board of formal notice and demand.