Purchase Price Tied to an Index?

[George Lewis]

Is it illegal to tie the purchase price of a house ( that you lease to a tenant/buyer) to a real estate index or an inflation index of the area?  It would seem it would make more sense to do that then try to estimate what the purchase price of the house (or similar houses) would be in 3 years.  I do understand the risk here to the landlord / owner if housing prices take a nose dive in the area.