Question about a promissory note as part of a purchase

[Brett S.]

I sold a rental property this year.  As I'm going through the paperwork to prepare for taxes, I noticed that about 20 years ago my tax accountant put the contractual sale price of the home onto the asset register and depreciated it accordingly.  However, I actually paid more than the closing paperwork says because the seller took a promissory note in parallel that is documented between us but not with the transaction paperwork.

I'm guessing I'd have to include it as an "improvement", if anything to get that into the basis for the house, since I can't go back and restate taxes after 3 years...  and risk an audit.

So I might just be out of luck.

Any thoughts?