Legal Question in Real Estate Law in California

Perhaps. There isn't quite enough information given in your question, such as why you relied upon an oral agreement to commit $30,000 to someone else's real property, to give you a "for sure" answer, but I think you may have a case based upon the equitable theory of "unjust enrichment." If all the facts show that you innocently and unintentionally benefitted someone else, you may have an enforceable claim for restitution. Your case would be strengthened if there is some written evidence of the "verbal" (lawyers say "oral") purchase agreement and other facts. It may also be important to know how and why the owner voided the original agreement. To win a case based on an equitable theory, you also need to show that you were not significantly at fault or trying something tricky (the "clean hands" doctrine). Have a discussion with a local lawyer.