No, you are not required to agree to an extension. That said, if the delay is just a few days, you would likely be obligated to wait for the new close, rather than terminating the contract, since a delay of a few days would not be considered sufficiently "material," or important, as to justify contract termination unless you had previously (in writing) made it clear that "time was of the essence" and you had a non-negotiable hard deadline. You may be able to seek compensation for the extra costs the delay caused, however: if they won't make an adustment at closing, you could in theory sue later for the money. A delay of a couple of weeks or more should let you treat the contract as terminated by the other side's breach, however.