Notwithstanding any other provision of law, the Administrator of General Services, in consultation with the Director of the Office of Government Ethics, shall prescribe by regulation the conditions under which an agency in the executive branch (including an independent agency) may accept payment, or authorize an employee of such agency to accept payment on the agency’s behalf, from non-Federal sources for travel, subsistence, and related expenses with respect to attendance of the employee (or the spouse of such employee) at any meeting or similar function relating to the official duties of the employee. Any cash payment so accepted shall be credited to the appropriation applicable to such expenses. In the case of a payment in kind so accepted, a pro rata reduction shall be made in any entitlement of the employee to payment from the Government for such expenses.