29 C.F.R. § 20.51 – Exemptions.

(a) The provisions of 31 U.S.C. 3717 do not apply:
1. (1) To debts owed by any State or local government;
2. (2) To debts arising under contracts which were executed prior to, and were in effect on (i.e., were not completed as of), October 25, 1982;
3. (3) To debts where an applicable statute, regulation required by statute, loan agreement, or contract either prohibits such charges or explicitly fixes the charges that apply to the debts involved; or
4. (4) To debts arising under the Social Security Act, the Internal Revenue Code of 1954, or the tariff laws of the United States.
(b) Agencies are authorized to assess interest and related charges on debts which are not subject to 31 U.S.C. 3717 to the extent authorized under the common law or other applicable statutory authority.