5 C.F.R. § 919.305
May I enter into a covered transaction with an excluded or disqualified person?
(a) You as a participant may not enter into a covered transaction with an excluded person, unless the OPM grants an exception under § 919.120.
(b) You may not enter into any transaction with a person who is disqualified from that transaction, unless you have obtained an exception under the disqualifying statute, Executive order, or regulation.