The listing agent calls you regarding your low offer. They ask what your buyer's real number is, and if the sellers counter, how much more your buyer will come up with. In your buyer's best interest, you tell the listing agent:

To counter, and you will have to also present it to your buyer

Explanation:

Going by the question you have a fiduciary duty to your client and can't just disclose confidential information to the other agent. By revealing those confidential information you are breaching the terms on client private data confidentiality and trust which can get you sued by your client.