Short answer. Yes. Article 1472 treats the price as certain when it is fixed as what the thing sold would have on a definite day, or in a particular exchange or market, or a set amount above or below that price, provided the amount is certain. The price need not be a fixed number today.
What the law says
the price fixed is that which the thing sold would have on a definite day, or in a particular exchange or market, or when an amount is fixed above or below the price on such day, or in such exchange or market, provided said amount be certain
Civil Code, Article 1472 — Price by Reference to an Exchange or Market. Read the full provision →
A price can be certain without a fixed number
For a sale to be valid the price must be certain, but certainty does not mean you must name a peso figure when you sign. Article 1472 allows the price of securities, grain, liquids, and other things to be considered certain when it is tied to an external, verifiable reference instead of a stated amount. The provision recognizes that for goods traded on exchanges or open markets, the fair value moves day to day, and parties often want the price to track that value rather than gamble on a number set in advance. So pegging your price to a market or exchange rate on a future date is a recognized, legitimate way to keep the price certain.
How the reference must be framed
The article lists the acceptable formulas. The price is certain when it is that which the thing sold would have on a definite day, or in a particular exchange or market, or when an amount is fixed above or below the price on such day, or in such exchange or market, provided said amount be certain. Two features matter. First, the reference point must be identifiable — a definite day and a particular exchange or market, not a vague gesture at 'the going rate' with no source. Second, if you add a margin — so many pesos or a set figure above or below that reference — that add-on amount must itself be certain. A clearly defined reference plus a definite margin keeps the whole price legally certain.
Why this works and where care is needed
The logic is that a price is certain if it can be made definite by simple reckoning, without the parties having to negotiate again. Once the agreed day arrives and the market or exchange figure is known, the price becomes a matter of calculation, not further agreement. The practical caution is precision: name the exact market or exchange and the exact day, and if you use a margin, state it exactly. Ambiguity about which market, which date, or how much to add or subtract is what invites disputes over whether the price was ever truly certain. Drafted with that care, a future-market or exchange-rate price is fully valid under this article.