Short answer. You can recover, but only to the extent the debtor actually benefited from your payment. Article 2050 of the Civil Code applies the rules on third-party payment to this situation: if you paid without the debtor's knowledge, you can claim reimbursement only for the amount that was actually beneficial to him.
What the law says
If a guaranty is entered into without the knowledge or consent, or against the will of the principal debtor, the provisions of articles 1236 and 1237 shall apply.
Civil Code, Article 2050 — Guaranty Without the Debtor's Knowledge. Read the full provision →
What the statute says
Article 2050 of the Civil Code addresses guaranties that the debtor never agreed to: "If a guaranty is entered into without the knowledge or consent, or against the will of the principal debtor, the provisions of articles 1236 and 1237 shall apply." These two articles govern what happens when any third person pays on behalf of a debtor without that debtor's involvement. By pointing to them, Article 2050 makes clear that a secret guaranty is legally valid — the creditor is protected — but the guarantor's recovery rights are limited compared to a guarantor the debtor actually authorized.
The benefit-of-the-debtor limit under Article 1236
Article 1236 sets the ceiling on what you can recover in this situation. It provides that if you paid without the debtor's knowledge or against his will, you "can recover only insofar as the payment has been beneficial to the debtor." What counts as a benefit? The standard measure is the amount by which the debtor's debt was actually reduced — the portion of the creditor's claim that was extinguished by your payment. If the debtor had a valid defense that would have reduced the debt, and you paid the full amount without knowing about it, you may not recover the excess from the debtor.
No right to subrogation under Article 1237
Article 1237 adds another restriction: without the debtor's knowledge or consent, you cannot compel the creditor to subrogate you in his rights — such as those arising from a mortgage, guaranty, or penalty. Subrogation means stepping into the creditor's shoes and inheriting his security rights against the debtor. A guarantor who was authorized by the debtor can demand this substitution; one who acted without the debtor's knowledge cannot. This matters when the loan was secured by real estate or other collateral — you would not be entitled to foreclose or enforce that security in the debtor's place.
What this means for you practically
If you pay the creditor and the debtor benefits fully — the entire loan is settled and the debt is gone — you can generally recover the full amount you paid, because the full amount was beneficial. If there were offsets, defenses, or partial payments the debtor already made that you were unaware of, your recovery may be less than what you paid out. Before paying any amount as guarantor under a guaranty the debtor never knew about, it is worth notifying the debtor first and, if possible, getting his acknowledgment — that converts the situation from one governed by Articles 1236 and 1237 into a fully authorized arrangement where your recovery rights are much cleaner.