Mechanics of buying a stake in a friend's condo

[Bob Harlow]

Hi,

I am interested in buying a stake in a friend's condo. Currently, the condo is worth about $400,000 and there is a mortgage of about $300,000 on it. He is interested in buying a new property to move in to with his significant other, but they need some additional money for their down payment on the new property. Rather than just issuing them a loan of $100,000 for the down payment on the new house, I would rather buy a stake in the existing condo. How would one go about structuring this? Ideally, I'd like to have a situation where the mortgage remains in his name and the title of the house is transferred to an LLC where I own 25% and he owns 75%. Is there a way to pull this off so I buy into the LLC and he gets the $100,000 cash when the LLC is set up? The goal here would be to split all profits and non-mortgage expenses 25/75 (he is responsible for mortgage expenses and will handle the interest deduction on his personal tax return).

Lets assume for the time being that he is quite credit worthy, the lender is okay with the title transfer if he provides a personal guarantee and I am okay with the external lien on the property. 

Thanks!