This happened to my dad once…there was some out of town realtor who sold for below market and every thing was signed…The owner then wanted to back out since it was lower than they wanted… but I think my dad forced the sale since he already had all your t’s/i’s crossed, dotted/signed, etc…(valid contract)…
Like it was mentioned, you can force the sale if all the contract stuff was done, but you should probably consult an attorney…anything verbal is not legally binding.