You need to ask your real estate agent who's representing you on this deal and who's familiar with the documents you're referring to.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
The exact answer will depend upon the real estate purchase agreement signed by buyer and seller. Normally, a cancellation like this, will not be considered valid until it is received. You should consult with a real estate attorney, who can review the contract, in order to get an exact response.