You potentially have a valid claim for breach of contract. It depends on what the original listing agreement says about amending the agreement. It may be that the original listing agreement requires all parties to sign a modification (such as an amendment to sell the property at a reduced price) to be validly amended. On the other hand, in some circumstances, email communications can be binding and enforceable as a writing, particularly where the email contains the full name of the party to be charged. You should hire an attorney to review your documents and conduct limited legal research to properly evaluate the strength and weaknesses of your claims. You might have a good case.
I am not a CA attorney, laws vary from state to state, therefore you should always consult a local attorney.
Here in NJ, I represent real estate brokers and have successfully sued for commissions.
The law here is that if the broker presents a ready, willing and able buyer at the asking price (with no requests to change the contract of sale), then the seller is required to accept the offer - and the seller would be liable to the broker for the commission for failing to complete the transaction.
You should consult a CA attorney who is familiar with brokerage law.
If this answer was helpful, please mark it as helpful or as a best answer. This answer is for general education purposes only. It neither creates an attorney-client relationship nor provides legal guidance or advice. The answer is based on the limited information provided and the answer might be different had additional information been provided. You should consult an attorney.
If your agreement provides for arbitration, use it. That's what it is for. Much easier and faster than litigation. Otherwise you'll have to start the "legal" process with an attorney letter outlining the facts and demand for payment. If there is any question about the authority to reduce price, expect that defense.
The foregoing is for informational purposes only and may not be relied on as attorney-client advice.