Hello and thanks for using Just Answer.
I am Al and am happy to assist you with your enquiry.
I am afraid that only once Contracts have been signed and formally exchanged between the parties, via their respective Solicitors, is anything legally binding on either party.
As such, although you may have received an offer in writing, it still remains "subject to Contract" until the Solicitors have formally exchanged Contracts.
Therefore, as it stands, either party is free to withdraw from the transaction without any penalty.
I am sorry this is not the answer you were looking for, but it sets out the legal position.