Neither of those scenarios would be enough to give rise to an estoppel. This situation probably lacks the element of inducement that gives rise to unconscionability, e.g. it'd be different if the vendor induced the buyer to believe they'd be released from the subject contract, in order for the vendor to sell them the second cheaper property, and then tried to sue for default on the first contract.
It really is hugely unlikely Mamaof3 went anywhere near creating an estoppel, I'm just making the point that you have to be careful in what you say.
A good summary of the principles of estoppel can be found here: http://www.australiancontractlaw.com/cases/walton.html
Once the vendor's verbally accepted the buyer's request to terminate by mutual agreement, it's a done deal - they can't change their mind in writing later. The contract discharges at that moment. Written contracts can only be varied in writing, but can be terminated orally. < Edit: this is at common law; there may be statutory requirements that termination is in writing, I'm unsure. In any case, it's obviously good practice to have it in writing.