I'm not a lawyer and I don't know what the law states in this particular instance, but my gut tells me to make out the bill of sale to the buyer stating on the bill of sale the steps of possession that the gun will go through including using the FFL holder as the intermediary and listing that intermediary FFL holder's address, business name, and other relevant information you have to confirm that it is in fact a FFL location.
Sold firearm xzy, serial number 123456789, to Joe Public on such and such a date, with transfer to take place via John Q Public's Guns 'n Grub, 13579 N. Main Street, New World City, AnyState, zip 98765-4321, FFL # 12a34b56c, tel: (123)555-1212, fax: (123)555-1212.
Or words to that effect. But check with someone else and don't listen to me as I know nothing.