This is classic offer and acceptance in contract law. If you did indeed offer, in writing, $50k for that person's house in something that wasn't a joke offer, and then other person accepted for whatever reason before you rescinded the offer, that's a valid contract for sale of something. There may be additional concerns given it's real property and not goods, but the point stands: the owner was presented with an offer, then countered the offer with $50k, and the first party accepted the offer. All in writing. This creates a valid contract.