Selling agent signed the counter offer, is it legal

[Jo Zhou]

Hi, I am in the process of negotiating a contract to buy a house. I live in California. Today, the counter offer was back with signature said "per phone conversation with seller", signed by seller's agent. I wonder if this counter offer is valid. If I signed the counter offer, is it a legal, enforceable contract?

I worried if seller comes back and said he/she didn't say that, what will happen?

Thank you.