To answer Daniel's reply, money now is worth more than money later. Every 10K drop in sale price is better for you and worse for him due to inflation and risk of nonpayment. The clause mentioned is a prepayment penalty, where an extra percentage is paid to make up for it.
BTW, any of the scenarios above are very generous to the buyer in the seller finance field. For the seller to create such an offer for you would require high quality creditworthiness...
To Josh, if you wish for standard mortgage terms and are 50K apart in pricing, I do not foresee a deal. Why would he want to take a ~20% price cut to only get his full price over time? For him to be this motivated, he would reasonably wish to be paid all at once through cash or bank financing.
If you wish for him to take a SF deal, you'll need to be prepared to create a valuable note from his perspective. PM if you wish and I can send over some general guidelines to help protect his investment and perhaps get you closer to a reasonable deal.