Short answer. Not automatically — the choice is yours. An act beyond the authority you gave does not bind you unless you adopt it. But once you ratify, or even signal that you are willing to, the other party can no longer escape the deal by pointing at your agent's lack of authority.
What the law says
A third person cannot set up the fact that the agent has exceeded his powers, if the principal has ratified, or has signified his willingness to ratify the agent's acts.
Civil Code, Article 1901 — Third Person and Ratification. Read the full provision →
What the article settles
Article 1901 deals with the moment after the overstep: A third person cannot set up the fact that the agent has exceeded his powers, if the principal has ratified, or has signified his willingness to ratify the agent's acts. It assumes what most people already sense — that an act beyond the authority given does not by itself bind the principal, and that the decision whether to accept it belongs to him. What the provision denies is the other side's attempt to use your agent's excess as an escape hatch after you have chosen to accept. The outsider does not get to change his mind because your agent overreached.
Ratification can be words or conduct
You can ratify in so many words, but the article reaches further: it is enough that you have signified your willingness to ratify. In practice, conduct speaks loudest. Accepting the price, delivering the goods, cashing the payment, moving into the property, telling the buyer to go ahead — any of these can amount to adopting the act. That is why silence and convenience are dangerous. A principal who is privately unhappy about the excess but quietly takes the benefit is very likely to be treated as having accepted it, and will not be heard afterwards to say the agent had no power.
You cannot take the good and leave the rest
Ratification attaches to the act as a whole. You cannot approve the sale but disown the discount your agent threw in, or keep the deposit while rejecting the delivery date he agreed to. If the terms are unacceptable, the answer is to reject the act, promptly and in writing, rather than to accept parts of it. Note also what the article does not do. It does not make lawful an act that is unlawful in itself, and it does not settle what your agent owes you for exceeding his instructions — overstepping can leave him answerable to you even after you have ratified.
If you do not want to be bound
Move quickly. Tell the other party in writing that the agent had no authority for what he did and that you do not adopt it, and take no benefit from the transaction while the position is unresolved — money received should not be spent. Where the authority was written, give a copy so the outsider can see its limits, and if it was filed or registered anywhere, correct that record too. The other party may still have claims against the agent personally. These disputes turn on what was said and done at the time, so keep the messages and take advice early.