Federal ethics laws (31 USC 1353 and 5 USC 4111 - see below for statutes) pertaining to gifts of travel support from nonfederal sources generally require that nonfederal travel support must be directed to the agency (rather than directly to the employee), and payment to the employee must be in-kind (such as an airline ticket).

However, a statutory exception allows 501(c)(3) organizations to make direct payments of travel support to federal employees. 5 USC 4111 states:

(a) To the extent authorized by regulation of the President, contributions and awards incident to training in non-Government facilities, and payment of travel, subsistence, and other expenses incident to attendance at meetings, may be made to and accepted by an employee, without regard to section 209 of Title 18, if the contributions, awards, and payments are made by an organization determined by the Secretary of the Treasury to be an organization described by section 501(c)(3) of title 26 which is exempt from taxation under section 501(a) of title 26.

(b) When a contribution, award, or payment, in cash or in kind, is made to an employee for travel, subsistence, or other expenses under subsection (a) of this section, an appropriate reduction, under regulations of the President, shall be made from payment by the Government to the employee for travel, subsistence, or other expenses incident to training in a non-Government facility or to attendance at a meeting.

Similarly, VA MP-1, Part II, Chapter 2, Paragraph 3(j)(1) provides additional guidance:

Under the authority of 5 USC 4111, an organization which is private and nonprofit as described in 26 USC 501(c)(3) and obtains tax exempt status under 26 USC 501, may pay directly to an employee the travel expenses associated with training or the attendance at meetings. However, as required by 5 USC 4111 (b) and 41 CFR 304 the amounts that otherwise would be payable by the Government on account of the employee's travel, or for subsistence, will be reduced by the amount of contribution or payment which is made by the private organization for travel, subsistence or other expenses.

Finally, it should be noted that a nonprofit may provide reimbursement for travel conducted on "off duty" status (annual leave) as well as "on duty" status (official absence, authorized absence, administrative leave) provided that the purpose of the travel supports an agency mission or an otherwise allowable activity.  That is, an NPC may provide reimbursement for travel related to outside speaking, teaching and writing regardless of duty status, provided that the documented purpose of the travel supports VA's research or education mission.  However, travel conducted on annual leave should be discouraged and requires particular attention to justification and documentation.

Regardless of duty status, federal employees traveling on non-federal travel support are responsible for accepting travel reimbursement in accordance with federal laws, regulations and VA policies regarding acceptance of non-federal travel support under section 1353 of title 31, U.S.C. or section 4111 of title 5, U.S.C.  VA employees are also responsible for completing all forms required by VA and for obtaining VA approvals related to the travel.

TITLE 31 > SUBTITLE II > CHAPTER 13 > SUBCHAPTER III > § 1353

§ 1353. Acceptance of travel and related expenses from non-Federal sources

(a) 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.

(b) Except as provided in this section or section 4111 or 7342 of title 5, an agency or employee may not accept payment for expenses referred to in subsection (a). An employee who accepts any payment in violation of the preceding sentence—

(1) may be required, in addition to any penalty provided by law, to repay, for deposit in the general fund of the Treasury, an amount equal to the amount of the payment so accepted; and

(2) in the case of a repayment under paragraph (1), shall not be entitled to any payment from the Government for such expenses.

(c) As used in this section—

(1) the term “executive branch” means all executive agencies (as such term is defined in section 105 of title 5); and

(2) the term “employee in the executive branch” means—

(A) an appointed officer or employee in the executive branch; and

(B) an expert or consultant in the executive branch, under section 3109 of title 5; and

(3) the term “payment” means a payment or reimbursement, in cash or in kind.

(d)

(1) The head of each agency of the executive branch shall, in the manner provided in paragraph (2), submit to the Director of the Office of Government Ethics reports of payments of more than $250 accepted under this section with respect to employees of the agency. The Director shall make such reports available for public inspection and copying.

(2) The reports required by paragraph (1) shall, with respect to each payment—

(A) specify the amount and method of payment, the name of the person making the payment, the name of the employee, the nature of the meeting or similar function, the time and place of travel, the nature of the expenses, and such other information as the Administrator of General Services may prescribe by regulation under subsection (a);

(B) be submitted not later than May 31 of each year with respect to payments in the preceding period beginning on October 1 and ending on March 31; and

(C) be submitted not later than November 30 of each year with respect to payments in the preceding period beginning on April 1 and ending on September 30.

TITLE 5 > PART III > Subpart C > CHAPTER 41 > § 4111

§ 4111. Acceptance of contributions, awards, and other payments

(a) To the extent authorized by regulation of the President, contributions and awards incident to training in non-Government facilities, and payment of travel, subsistence, and other expenses incident to attendance at meetings, may be made to and accepted by an employee, without regard to section 209 of title 18, if the contributions, awards, and payments are made by an organization determined by the Secretary of the Treasury to be an organization described by section 501 (c)(3) of title 26 which is exempt from taxation under section 501 (a) of title 26.

(b) When a contribution, award, or payment, in cash or in kind, is made to an employee for travel, subsistence, or other expenses under subsection (a) of this section, an appropriate reduction, under regulations of the President, shall be made from payment by the Government to the employee for travel, subsistence, or other expenses incident to training in a non-Government facility or to attendance at a meeting.