Short answer. There is a presumption in your favor. Article 1272 of the Civil Code presumes that when the private document evidencing a debt is found in the debtor's possession, the creditor delivered it voluntarily, which supports treating the debt as remitted. The creditor may still rebut this by proving otherwise.
What the law says
Whenever the private document in which the debt appears is found in the possession of the debtor, it shall be presumed that the creditor delivered it voluntarily, unless the contrary is proved.
Civil Code, Article 1272 — Presumption From the Debtor's Possession of the Document. Read the full provision →
What the presumption actually says
Article 1272 addresses a private document, meaning one that is not notarized or otherwise public, in which a debt is recorded, such as an IOU. When that document turns up in the debtor's possession, the law presumes the creditor delivered it voluntarily. Because a creditor who still expects payment ordinarily has little reason to hand over the very paper proving the debt, this presumption of voluntary delivery supports the further conclusion, under the Civil Code's rules on remission, that the obligation itself was forgiven.
Voluntary delivery, and what it implies
The presumption is specifically about voluntary delivery, not accidental loss, theft, or the document being returned for some other reason. If the creditor gave you the IOU willingly and without qualification, the natural inference is that they intended to give up their claim under it. This links Article 1272 to how the Civil Code treats renunciation of a debt more generally, where a creditor's voluntary act of surrendering the evidence of the debt is treated as meaningful, though not conclusive, proof that the debt was forgiven.
The creditor can still rebut it
Article 1272 ends with unless the contrary is proved, which makes this a disputable presumption rather than an automatic discharge. A creditor who insists you still owe the money will need to show the document reached your hands some other way, for example that it was lost, stolen, borrowed temporarily, or given to you for a purpose unrelated to forgiving the debt. Until the creditor produces that kind of contrary proof, the presumption favors you as the debtor now holding the document.
What kind of evidence strengthens your position
Because the presumption can be rebutted, it helps to be able to show the circumstances of how the document reached you, beyond simply having it in hand. Any message, note, or conduct from the creditor around the time the document was returned that suggests they meant to release the debt makes the presumption harder to dislodge. Conversely, if you know the document changed hands for another reason, such as being handed to you temporarily, keeping that context in mind before assuming the debt is fully settled protects you from relying on a presumption a creditor may later be able to overcome.