41 C.F.R. § 102-118.180

A TSP may file a transportation claim against an agency under 31 U.S.C. 3726 for—

(a) Amounts owed but not included in the original billing;

(b) Amounts deducted or set off by an agency that are disputed by the TSP; or

(c) Requests by a TSP for amounts previously refunded in error by that TSP.