Short answer. No. Article 1435 of the Civil Code stops a person who sold or alienated a thing in representation of another from later claiming ownership of that same thing against the buyer. Having sold it as someone else's agent, he is barred from asserting his own title against you.
What the law says
If a person in representation of another sells or alienates a thing, the former cannot subsequently set up his own title as against the buyer or grantee.
Civil Code, Article 1435 — Agent's Estoppel. Read the full provision →
What this rule prevents
Article 1435 addresses a specific kind of unfairness: someone sells property while representing that it belongs to another person, collects the price or otherwise completes the sale on that basis, and then turns around and claims the property was actually his own all along. The article closes that door directly, saying the seller cannot subsequently set up his own title as against the buyer or grantee. Having sold the thing in a representative capacity, he is bound by the position he took at the time of the sale.
Why it does not matter whether he actually owned it
Even if it later turns out the seller genuinely did have some title to the thing, separate from his role representing another person, Article 1435 still bars him from raising that title against you as the buyer. The rule is not about whether his hidden ownership claim is true or false; it is about preventing him from contradicting the very basis on which he sold you the property in the first place, which was that he was acting for someone else, not for himself.
What this means for you as the buyer
As the buyer, you are entitled to rely on the position the seller himself took at the time of the transaction. If he represented that he was selling on another's behalf, you do not need to independently defend your title against a later claim of ownership from that same seller; Article 1435 does the work for you by barring him from making that claim at all. This does not resolve every possible dispute over the property, such as a claim from the actual principal or a third party, but it forecloses the seller's own reversal.
How this connects to the law on agency
Article 1435 reflects a principle that runs throughout the Civil Code's treatment of agents and representatives: a person who deals with others on someone else's behalf cannot later disown that role when it becomes inconvenient. Estoppel of this kind protects buyers who reasonably relied on how a transaction was presented to them at the time. Whether the seller was acting as a formal agent, an administrator, or simply someone representing that the property belonged to another, the same principle applies once he actually sold the thing on that stated basis.