Hehe.. ok I'll clarify it a bit more. I thought by using A and B I'd make it impartial, but that's too confusing, sorry guys. A thinks he's clever.
I want to do this because my parents are getting on in years and eventually will require more support from us.. Rather than wait and have to pay for renting them a place to live (eventually), I figure if they do something now it'll be less of a financial cost in the future. In 10 years they can have enough equity built up that they could have pretty low monthly payments.
I don't
really care if I never get the money back, but as Cow pointed out it's good to have something in writing so I wanted to come up with something to sign with my parents. ^_^
EDIT: It'd be nice if they could save up a down payment themselves, but I don't ever see that happening. I'd buy the house myself and rent it to them but with my wife on maternity leave I can't do that at the moment.
Some ideas we had were split the "profit" when the house is sold by the percentage the down payment is with respect to the value of the house at purchase time.. IE if we put down 5%, then we get 5% of the "profit". So if the house increases 10% / year, our ROI would be 5% per year.