The chain of documents from the origin of the money to the business account: the sale agreement and statement of adjustments, the lawyer's trust statement, the deposit into the applicants' account, and the transfers out to the US enterprise. Each step should be traceable without explanation from the applicants.
Build the trail before the money moves
A source-of-funds file is easiest to assemble while the transaction is happening and hardest to reconstruct a year later. Keep the listing and sale documents, the closing statement showing the net proceeds, and the bank record of the deposit. If the property was mortgaged, keep the discharge statement so the net figure is explained.
When funds are transferred to the United States, transfer them from the account they landed in rather than moving them through several accounts, and keep every wire confirmation. If any part of the money is borrowed, record what secures the loan, since funds secured against the assets of the business being acquired are not treated as the investor's own capital at risk. If part is a gift from a relative, document the gift and that relative's own source of the money.
Hypothetical example: a custom-orthotics lab buyer signs an asset agreement, pays a deposit, and assumes the full purchase price is proven by the agreement. The payment trail and the agreement's refund clauses determine what has actually been committed. Assemble bank statements, wire confirmations, invoices, lease documents, licences, ownership records, and a business plan tied to source figures.
A first review should label each expense as recoverable or exposed to loss. Missing proof of payment or a refund right that has not been explained can undermine an otherwise sound acquisition.