Legal Question in Real Estate Law in California

No, the agent can not by themselves change the sales price. If the buyer reasonably does not notice the change, there is no contract of sale at the new price. If the buyer does notice, there is still some question of the validity as the terms are not supposed to be changed at escrow. But since yo had to deposit the total purchase price before escrow closed, I do not see how the price could have been changed.

The escrow officer must follow the joint escrow instructions. In some situations, the escrow officer may have those instructions unilaterally amended, but only by the party to the escrow.

My answer is basically going to be no, just as Messrs. Shers and Roach have said, but more facts including the timing (relative to signing the documents before the notary, who was present, why the agent did this, and several related facts to describe the situation more fully would be needed in order to tell you whether this was an innocent attempt to rectify a mistake, a bold attempt to raise the price, a sneaky and possibily criminal attempt to commit a fraud, or something else. One thing in particular that causes me to wonder if we have all the facts is that you say this happened "at the signing" and yet it took a phone call to the title company to have the changes made. What about the papers that were present at the signing?