29 C.F.R. § 4903.20
Will PBGC issue a refund if money is erroneously collected on a debt?
eCFR amendment effective 2017-01-01 · current text
PBGC will promptly refund to a debtor any amount collected on a debt owed to PBGC when the debt is waived or otherwise found not to be owed to the United States, or as otherwise required by law.