29 C.F.R. § 4219.31 : Labor — REGULATIONS RELATING TO LABOR — PENSION BENEFIT GUARANTY CORPORATION — WITHDRAWAL LIABILITY FOR MULTIEMPLOYER PLANS — NOTICE, COLLECTION, AND REDETERMINATION OF WITHDRAWAL LIABILITY — OVERDUE, DEFAULTED, AND OVERPAID WITHDRAWAL LIABILITY — Overdue and defaulted withdrawal liability; overpayment

(a) Overdue withdrawal liability payment. Except as otherwise provided in rules adopted by the plan in accordance with §4219.33, a withdrawal liability payment is overdue if it is not paid on the date set forth in the schedule of payments established by the plan sponsor.
(b) Default.
(1) Except as provided in paragraph (c)(1), "default" means-
(i) The failure of an employer to pay any overdue withdrawal liability payment within 60 days after the employer receives written notification from the plan sponsor that the payment is overdue; and
(ii) Any other event described in rules adopted by the plan which indicates a substantial likelihood that an employer will be unable to pay its withdrawal liability.
(2) In the event of a default, a plan sponsor may require immediate payment of all or a portion of the outstanding amount of an employer's withdrawal liability, plus interest. In the event that the plan sponsor accelerates only a portion of the outstanding amount of an employer's withdrawal liability, the plan sponsor shall establish a new schedule of payments for the remaining amount of the employer's withdrawal liability.
(c) Plan review or arbitration of liability determination. The following rules shall apply with respect to the obligation to make withdrawal liability payments during the period for plan review and arbitration and with respect to the failure to make such payments:
(1) A default as a result of failure to make any payments shall not occur until the 61st day after the last of-
(i) Expiration of the period described in section 4219(b)(2)(A) of ERISA;
(ii) If the employer requests review under section 4219(b)(2)(A) of ERISA of the plan's withdrawal liability determination or the schedule of payments established by the plan, expiration of the period described in section 4221(a)(1) of ERISA for initiation of arbitration; or
(iii) If arbitration is timely initiated either by the plan, the employer or both, issuance of the arbitrator's decision.
(2) Any amounts due before the expiration of the period described in paragraph (c)(1) shall be paid in accordance with the schedule established by the plan sponsor. If a payment is not made when due under the schedule, the payment is overdue and interest shall accrue in accordance with the rules and at the same rate set forth in §4219.32.
(d) Overpayments. If the plan sponsor or an arbitrator determines that payments made in accordance with the schedule of payments established by the plan sponsor have resulted in an overpayment of withdrawal liability, the plan sponsor shall refund the overpayment, with interest, in a lump sum. The plan sponsor shall credit interest on the overpayment from the date of the overpayment to the date on which the overpayment is refunded to the employer at the same rate as the rate for overdue withdrawal liability payments, as established under §4219.32 or by the plan pursuant to §4219.33.

29 C.F.R. §4219.31