Several pieces of information are missing here. Do you have a signed and accepted offer? Do you have a Buyers agent who submitted the offer? If you have a buyers agent I would seek their help in rectifying this matter, otherwise if you can't work this out some other way, I would consult an attorney for their opinion. A seller can change their mind on selling no matter what the offer is, however if you have a signed agreement by both parties as to price and conditions, that is a different story. Best of Luck to you !!