Moving out was part of the deal for selling the house
Per your deal with the buyer? You MIGHT be able to sue for detrimental reliance if you signed a lease, but only a local attorney can tell you if it's possible.
You MIGHT convince a judge to modify, but no one can tell you if he'll agree.
However, you might have to realize that - in this economy - selling the home at a "steal" may be the only way to go, even if you have to lose $.