Short answer. You are not liable for the lost substitute; you simply deliver the main thing. Article 1206 says the loss or deterioration of the thing intended as a substitute, through the negligence of the obligor, does not render him liable, because only the main prestation was ever truly owed.
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.
Civil Code, Article 1206 — Facultative Obligations. Read the full provision →
Only the main prestation is really owed
In a facultative obligation, as Article 1206 describes, only one prestation has been agreed upon, but the obligor may render another in substitution. That single agreed prestation — the main thing — is the only thing you actually owe. The substitute is merely an option you hold, a way you are allowed to pay if you choose. Because the substitute is not itself the object of the obligation, its fate before you have chosen it does not change what you must deliver. The creditor was never entitled to the substitute; he was entitled to the main thing.
Losing the substitute costs you nothing
That is why 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. Even if you carelessly destroyed the item you might have handed over instead, you have destroyed only your own alternative, not something the creditor could demand. Your duty is unchanged: deliver the main prestation. The negligence that ruined the substitute is legally irrelevant to the creditor, because he had no right to that item in the first place. You lose the convenience of the option, nothing more.
The rule flips once you substitute
The protection lasts only until you exercise the option. Article 1206 continues that once the substitution has been made, the obligor is liable for the loss of the substitute on account of his delay, negligence or fraud. When you actually choose to give the substitute in place of the main thing, that substitute becomes what you owe. From that point its loss through your delay, negligence or fraud does make you answerable, just as with any thing due. So timing is everything: before substitution the option can perish freely, but after substitution it carries the full weight of the obligation.
How to think about your situation
Since the substitute was destroyed before you ever elected to use it, you remain bound only to deliver the main thing, and you owe nothing extra for the ruined item. Do not confuse this with an alternative obligation, where the debtor genuinely owes one of several prestations; that is governed by different rules. In a facultative obligation there is one thing owed and one optional substitute. Provided you can still deliver the main prestation, the negligent loss of the option you never exercised leaves your liability exactly where it was.