be careful

if you do not have to. Lots of partners / roomates have to sell their house at wrong time in the same situation -- usually within one or two years. Investment is another case.

If you have to, find a lawyer help you an agreement indicating the downpayment is yours and the way to split the gain if sell the house later. You can write down your partner will pay the monthly mortgage, but I think it could be a potensial problem here..... for your reference only.