41 C.F.R. § 304-3.16

(a) If an employee accepts a payment from a non-Federal source in violation of this part, they may be required, in addition to any other penalty provided by law and applicable regulations, to pay the general fund of the Treasury, an amount equal to any payment accepted; and

(b) For reimbursement under paragraph (a) of this section, an employee will not be entitled to any reimbursement from the Government for travel expenses that the payment was intended to cover.