A dated copy of the official page and FAQ as they read it, a written summary of the fee, gift and category conditions on that date, and every question sent to the official channel with its answer. Terms can change, and the file shows what the couple relied on.
Date-stamp everything, because the page can change
The Gold Card framework is administered under the official program and executive order rather than as a stand-alone statutory category, so its current procedures and family terms must be re-verified. The couple should keep a dated copy of the official page, a summary of the fee, gift, EB-1 or EB-2 classification and family provisions, and all official instructions or correspondence. The current page states that the DHS processing fee is nonrefundable and that the gift is requested after successful vetting.
Sale documents should be supplied if the official process requests them, but the published program materials should not be supplemented with an invented general source-of-funds rule. Finally, record which family members were included and why the adult daughter was not.