41 C.F.R. § 304-3.2
Types of acceptable non-Federal source payments
Agencies or employees may accept payments other than cash from a non-Federal source for all official travel expenses to attend a meeting of mutual interest, or any portion of those travel expenses mutually agreed upon between the agency and the non-Federal source. Agencies or employees may not accept payments for non-meeting travel under this part. However, agencies or employees may be able to accept payments under other authorities (see § 304-3.17).