All of the contracts are subject to final approval and the purchase price is guaranteed in writing. On a partial purchase the risks are similar to seasoned notes. The investor and the seller share these risks. The larger the down payment, the more the note is worth. Sell the property for full appraised value, after all you are offering attractive terms.
Most people are willing to pay full price if they can get attractive terms.
The interest rate of the originated note should be a percentage or two above prime. The shorter the term of the note, the higher the present value. A balloon payment combined with a longer amortization period can lower the monthly payments for the property buyer without lowering the present value that much. Little and nothing down deals can be accomplished by doing partial purchases. Work with the contract buyer to come up with several purchasing options.