Sounds like a mess to me, if teh seller still wants to sell and the buyer still wants to buy, try a short sale with the sellers lender, it may take a long time but if the tenant is already in the property than you have time. The key is was this signed 12-24 months ago or is it to close 12-24 months in the future. if it is in the future the seller should not agree to do something they can not fullfill. If it is already signed you may want to consult a real estate attorney. Good luck with your situation
Lease purchases in a declining market are a real disaster in the making. I would speak with your Designated Broker and Attorney. You have to think ahead in this type of structure and it goes way beyond a simple question and answer on trulia.