20 CFR 408.901: What is an underpayment?

(a) An underpayment can occur only with respect to a period for which you filed an application for benefits and met all conditions of eligibility for benefits.

(b) An underpayment is:

(1) Nonpayment, where payment was due but was not made; or

(2) Payment of less than the amount due for a period.

(c) For purposes of this section, payment has been made when certified by the Social Security Administration to the Department of the Treasury. Payment is not considered to have been made where payment has not been received by the designated payee, or where payment was returned.