In my view this is not a good deal for you, but GREAT FOR "THE MAN" and I am not sure your math makes sense.
Here is my more pessimistic (or conservative summary) [Devil's advocate] I hope you take this as it is intended... as constructive feedback. So you buy an asset for $100K and effectively beleive you will sell it for a $50K loss ($100K - $20K - $30K). To offset this loss, you would have the generate $50K of pretax earnings to breakeven over the life of the agreement ($3.3K in earnings a year). [actually, more since you wouldn't get the $30K until the 15th year]
AND don't forget, you also agreed to operate that asset for 15 years and generate $80K of after tax cash flows (since repayment isn't an expense) to buy down the note (to the benefit of the option holder). That is an average of more than $5.3K/per year (after tax) you would need to generate each year. On a $100K property, I would be suprise if this scenarios would cash flow. Assuming your note was interest-free, you would need a cap rate of about 10% to make this work. And interest isn't free.
You will note that from your example, you actually wouldn't want to pay off the debt. (there is no incentive to you) You would want to just hold the asset and take the cash flows. The option holder unless they had some reason to trust you would see the incentive you have to just not pay the mortgage and collect rents until the bank foreclosed. Afterall, the MAN foreclosing (calling the property) and the bank foreclosing are pretty similar economically to you.
In short, what you get is the ability to operate unlimited properties for the MAN, where you strive to repay the 80% loan on each property, only to have the MAN call the property away from you in the end.
And don't get confused by the money you perceive getting. All of it in this deal is pumped into financing a property that ultimately goes to the MAN.
I would prefer a partnership where you had some upside. The structure may not be broken, but the economics you get in the example sure are!
FYI - You don't need to recapture depreciation when you sell an asset at a loss. In this case, you wouldn't have to recapture. Also, if I did a deal like this, I would have the option require a notice period and 1031 Exchange cooporation language, although this tax loss wouldn't need it.