29 C.F.R. § 20.26
(a) If the statute under which waiver or administrative review is sought is mandatory, that is, if it prohibits the agency from collecting the debt prior to the agency's consideration of the request for waiver or review (see Califano v. Yamasaki, 442 U.S. 682 (1979)), then collection action must be suspended until either
(b) If the applicable waiver/review statute is permissive, that is, if it does not require all requests for waiver/review to be considered, and if it does not prohibit collection action pending consideration of a waiver/review request (for example, 5 U.S.C. 5584), collection action may be suspended pending agency action on a waiver/review request based upon appropriate consideration, on a case-by-case basis, as to whether: