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Heh. Ask him to actually guarantee it -- that is, structure the transaction as a loan yielding 10% (or 15%) with him fully liable for the principal and the interest. See if he agrees :-/ Don't forget to check that the counterparty (the borrower) has assets to pay you back.
There are financial securities called REITs (Real Estate Investment Trusts) which invest in property (sometimes commercial, sometimes rental, read the prospectus) and return the income to you less a haircut. As a sanity check you can take a look at how high returns do they provide.