A signed gift letter, their tax filings, the sale documents for the assets that produced the money, brokerage and bank statements, and confirmation that no repayment arrangement exists.
Document the donors as if they were applying
Treat the parents as a second applicant for source-of-funds purposes. The gift letter should identify the parties, the amount, the date, the source of the funds and the fact that the transfer is irrevocable, and should be signed by both donors. Behind it belong the documents that show where the money came from: the purchase and sale closing statements for the duplex, including the mortgage discharge if there was one; brokerage statements showing when the shares were acquired, what was paid and what the disposals realised; and tax filings for the years in which those transactions were reported. Bank statements must then connect each of those events to the account that funded the investment, without gaps and without unexplained deposits. Provide a one-page schedule mapping every deposit to its source document, numbered to match the exhibits.
Hypothetical example: a family sells a minority interest in a regional food distributor and gives part of the proceeds to an adult child for investment. A first review produces the corporate sale agreement, valuation materials, tax reporting, gift instrument and bank trail in chronological order. The records must show both the lawful source and the child's unrestricted ownership of the gifted amount. If the family expects repayment, retains a security interest or cannot document the sale proceeds, the transfer is not a simple gift for filing purposes. Resolve the financial arrangement before money enters the project account rather than attempting to explain it later.