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Is this investment opportunity problematic?
If they own the old house outright, they can mortgage it to you. In many jurisdictions this relieves you of the obligation to chase for payment, and of any worry that you won't get paid, because a transfer of ownership to the new owner cannot be registered until any charge against a property (ie. a mortgage) has been discharged. The cost of to your friends of setting up the mortgage will be less than the opulent interest they are offering you, and you will both have peace of mind. Even if the sale of the old house falls through, you will still be its mortgagee and still assured of repayment on any future sale (or even inheritance). Complications arise if the first property is mortgaged. Although second mortgages are possible (and rank behind first mortgages in priority of repayment) the first mortgagee generally has a veto on the creation of second mortgages.