The official program page and FAQ as they read on the day, saved with the date; the government's stated sequence of fee, vetting, gift and petition; the family pricing; and a written note of which EB category is proposed for you and why. Terms are described as subject to change.
Date-stamp the terms you are relying on
Because the framework is published on an official page rather than in a statute, the wording on the day you pay is the record of what you agreed to. Save a dated copy of the page and the FAQ, and note the fee, the gift amount, the point at which the gift becomes due, the family terms and any statement about refunds or changes. Keep the receipt for the processing fee and any correspondence about vetting.
Separately, obtain a written assessment of which immigrant category would carry the case, EB-1 or EB-2, and what evidence supports it, since the gift does not replace that eligibility; for a business owner the likely candidates are EB-2 exceptional ability in business or, if he holds an advanced degree, the advanced-degree route with a national-interest waiver. Add the cross-border tax memorandum and the family's civil documents. Treat the folder as the case file you would need if the terms changed after payment and you had to show what you relied on.