Options for selling a home for more than it's worth

I have a buyer interested in a home that I'm almost done renovating and the comps on the houses in the area are around 80,000.00. This house however is completely renovated...everything is new. New roof, ac, kitchen, bathrooms, all the floors and even the doors and door knobs. The only thing that isn't new is the outside block and the framing. She wants to buy the house for $99,000 is looking to finance. She already knows that the house might not appraise for that price and still wants to buy it at that price. Can we do a second note on the house at the closing to cover the difference? Is this common?