The loan agreement and security instruments, proof of ownership and value of the collateral, the lender's details, and the full banking trail from advance to investment.
The collateral has its own source-of-funds story
Borrowing does not remove the source-of-funds requirement; it adds a layer. A reviewer will want to see the loan agreement naming the investor as borrower, the security instruments identifying the charged assets, and evidence that those assets belong to the investor. That evidence in turn requires showing how the collateral was lawfully acquired, which for a business property may mean the purchase documents, the mortgage history and years of corporate financial statements and tax filings.
Then comes the money trail: the advance credited to a named account on a named date, any transfers between accounts, currency conversion records, and the payment to the enterprise or escrow. Keep the chain unbroken and indexed. The most common cause of a request for further evidence is a step that exists in the investor's memory but not in the file.