@Daniel Ryu
By "mortgage", I'm not referring to a loan. I mean an instrument you file in the county records to show there is a lien on the property. This prevents your partner from selling the property without paying off your interest.
Here's how I'm envisioning the transaction:
1. Your partner buys the property, deeded in his name, with a loan in his name.
2. Your partner sells you a contractual interest in the property. For example, you pay him an amount equal to half the cash he has invested in the deal, and the two of you agree to share equally in all net profits and losses from the property, and the net proceeds from a sale.
3. Your partner executes a mortgage on the property, which you file in the county records, giving you a lien on the property so that your partner cannot sell the property without your consent.
Note that your partner would have to come up with the entire down payment himself, and you would buy in after the loan closed. You don't want to commit mortgage fraud.
As @Ben Leybovich points out, my suggestion would be a "fractional interest transfer" and technically violate the due on sale clause. But I don't see any way the bank could find out about it.