Real estate contracts are an odd sort of creature. Most contracts require both parties to agree and sign before they are accepted, but partly because of the nature of real estate contracts – one party making an offer that he or she doesn’t know if the other will accept, with no negotiation ahead of time – real estate contracts make it very clear that an offer remains irrevocable for a set period of time. Once you have signed that contract, you are bound to it until the other party accepts or rejects it.
If you are making an offer, be sure you really mean it. The other party just might accept it.