Were they in their option period? As was stated there are many outs. If you were selling the house yourself you may not be aware of these. Your agent can go over the contract with you. I am assuming you are going this alone as this is a question your agent can answer for you.
This is just one of the many reasons people should use an experienced agent when buying a home. When sellers go it alone and are not knowledgeable it can be easy for the other parties agent to take advantage of the situation.
There are too many items missing from your question to give a thorough answer. Best of luck to you.
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