Short answer. Only if your agreement was actually set up that way. The Civil Code recognizes a facultative obligation, where one thing is owed but the debtor is given the option to deliver something else instead. Outside that specific arrangement, a debtor cannot simply swap in a different item without your consent.
What the law says
When only one prestation has been agreed upon, but the obligor may render another in substitution, the obligation is called facultative.
Civil Code, Article 1206 — Facultative Obligations. Read the full provision →
What the law says
The loss or deterioration of the thing intended as a substitute, through the negligence of the obligor, does not render him liable. But once the substitution has been made, the obligor is liable for the loss of the substitute on account of his delay, negligence or fraud.
Civil Code, Article 1206 — Facultative Obligations. Read the full provision →
The substitution right has to be built into the obligation
Article 1206 defines a facultative obligation as one where only one prestation has been agreed upon, but the obligor may render another in substitution. The key word is agreed. This is not a general power every debtor has to unilaterally change what they deliver — it is a specific structure the parties set up, where a single thing is actually owed, but the debtor has been given the option, by the terms of the obligation itself, to deliver something else instead.
What is actually owed stays the principal thing
Because only one prestation is agreed upon, the principal thing is what the debtor legally owes; the substitute is merely an option available to the debtor. This distinction has real consequences before substitution happens: the article says the loss or deterioration of the thing intended as a substitute, through the negligence of the obligor, does not render him liable. Since the substitute was never actually owed, damage to it before it is used does not itself create liability, because it was not the obligation's real object.
The liability picture changes once the substitution is made
The moment the debtor actually exercises the option and delivers the substitute, the analysis flips: once the substitution has been made, the obligor is liable for the loss of the substitute on account of his delay, negligence or fraud. Once the debtor has chosen to hand over the substitute in place of the original thing, it becomes the thing that matters for measuring the debtor's performance, and ordinary liability rules attach to it from that point forward.
What to check in your own situation
If someone who owes you is offering to hand over something different from what you agreed on, the threshold question is whether your original agreement actually gave them that option — an obligation is only facultative if the parties set it up that way from the start. If nothing in your agreement grants a substitution right, the debtor cannot impose a different item on you without your consent; you remain entitled to the thing that was actually promised.