Legally, you need to wait until the cancellation is signed or you get a court requiring them to cancel (or simply ordering the cancellation) because until you do, someone else (the buyer) could try to step forward with a colorable claim (one that cannot be dismissed out of hand) on the property and offer to complete the sale, which means that you cannot transfer it in good faith with an assurance you are transferring title. You need to get this potential claim resolved first. If they will not cancel, let them know you will sue and seek a court order requiring them to do so as well as compensation for you costs, expenses, and losses incurred due to their delay in cancelling--and if they still will not cancel, file the legal action.