Selling a partial to cash out of a deal- feasible or desirable?

[Amanda G.]

Say an investor has enough cash to buy a SFR outright and sell on an owner-financed note, either in a tax advantaged account or after holding for a year. They want to get all their cash out by selling a partial. How would I calculate that? Here are some typical numbers for my area to use for an example:

purchase $50k, sell at $70k, note at 9.9% interest with a 10% down for 150 payments

Thanks for helping me see the variables, to see if this is a viable option.