There are 2 ways to handle this - each with pros and cons. The 1st is to explain to the Seller at closing that instwead of you making the payments, your money partner will make them. Be sure to have everyone sign a Disclosure that they know what is going on as far as the ST and who is making payments and that you are out. The advantage is that this is clean and easy and you're done with the transaction. The con is that there is still no guarantee that the buyer will make the payments, and the Seller might still dreag you back into it (that's why it's good to have the Disclosure).
The other way is to place a mortgage for $1 on the property with the stips that the mortgage can not be paid off until the 1st mortgage is paid; any default on the 1st is a default on the 2nd; and the 1st and 2nd must be paid in full by some pre-determined date. The advantage is that if the Buyer doesn't make timely payments, you can foreclose, get the house back, and sell it again. Also, the Seller is never negatively impacted and your reputation remains sterling.
The disadvantage is that you would have to make the payments on the 1st until the foreclosure was complete.
The key to a ST deal is to have a good Disclosure signed by the Seller - and keeping the loan current.
Hope that helps.