A balance shows money exists, not where it came from. For each sale, assemble the ownership evidence, the executed agreement, the closing or settlement statement, and the deposit that matches it. Where prescribed historical tax records apply to your brother, obtain them too rather than assuming a donor is exempt.
Building one file per sale instead of one summary
Summaries invite questions; files answer them. Suppose your brother sold a vehicle, a minority interest in a company, and a piece of land. Three separate packets read far better than a single spreadsheet, because each amount can be followed from title to contract to deposit. Corporate records behind the company interest may themselves be relevant to eligibility, so request them while the counterparties are still reachable.
Ask the seller for payroll registers rather than a headcount summary, and compare them against scheduled hours.
Keep the chain continuous even where a step is commercially uninteresting, because the reviewer is tracing money rather than reading a narrative.