Lease option question: Market downturn

[Kim Horn]

Let's say I use a lease option on a house that I own. I set it up for the tenant/buyer to purchase the home after two years of occupancy at its current appraised value of $250K. Then the two years is up and the market value of the home sinks to $175K. The bank will be unlikely to lend based on the old appraised value. What happens to the contract that I have with the purchaser of them home if the market value drops significantly?

Thanks!