Appreciation and the land contract

Hello, to all that view this post. I have a question about the land contract and appreciation. If an investor sells a house on a land contract and the selling price is based upon appreciation for houses in the area in the next two years, how is this appreciation determined? Also, is the notion of appreciation involves a certain level of uncertainty or speculation because real estate does not always appreciate in value?
If this is the case that real estate does not always appreciate or increase in value, can a lack of appreciation pose a problem to an investor in the land contract scenario?