How to get out of a contract for deed without legal ramifications?

I have a contract for deed agreement which was executed in June of 2006, with a two year option to buy which expired in June of 2008. Now with the market we are in, we have no desire to buy the property at the original purchase price agreed upon. The property is essed at $40k-$50k below original purchase price. Since we have fulfilled the original 2yr option to buy agreement without default the seller wants to amend the contract for another 2yrs. Are we obligated to do so, or can we walk away without a lawsuit?