Written for Mexico. This analysis applies to Mexican federal tax law — ISR (income tax), IVA (VAT) and SAT rules — and cites Mexican statutes. Amounts are in Mexican pesos (MXN).
"At what exchange rate?" is the question that jams contracts, invoices and dollar payments every day — and it has a precise legal answer almost no one knows in full. Mexico has an exchange rate with legal effects: the one the Bank of Mexico publishes in the DOF (the federal official gazette). Knowing which one it is, where to check it and when it's mandatory heads off disputes over cents that turn into fights over thousands.
The FIX and its publication in the DOF
Every business day Banxico sets the FIX exchange rate — an average of wholesale foreign-exchange market quotes — and publishes it in the DOF on the following bank business day. That published rate is, by mandate of art. 8 of the Monetary Law, the one applicable to settle foreign-currency obligations payable in Mexico: whoever owes dollars here can discharge the debt by paying pesos at the exchange rate in force (published in the DOF) on the payment date. Direct lookup: the DOF indicators and the Banxico portal — both linked with an explanation in our Resource Center.
Where it applies to you, case by case
- Dollar contracts (leases, sales, loans): the parties may agree on whatever exchange rate they want — bank X, date Y, average Z. If the contract is silent, the legal rule applies: the DOF on the payment date. The drafting lesson: always agree on the mechanism (which rate, from which date, and what happens on a non-business day) — 90% of exchange-rate disputes are contracts that didn't.
- Taxes (art. 20 CFF): taxes and their ancillary charges denominated in foreign currency are paid in pesos at the exchange rate of the day before the one on which they accrue — published by Banxico in the DOF. It's the rule that governs foreign-trade operations, withholdings from non-residents and any tax base denominated in foreign currency.
- Your accounting and ISR: exchange-rate fluctuation generates a gain or loss treated as interest (art. 8 LISR), and the law itself anchors the valuation to Banxico's exchange rate published in the DOF. Invoicing, provisioning and valuing with "my currency exchange's rate" produces differences the authority detects on cross-check.
- CFDI in dollars: it's issued with the corresponding exchange rate under the completion rules — the consistency among invoice, accounting and payment is exactly what electronic audits compare.
Do you have dollarized leases, loans or contracts?
For portfolios with dollar flows — dollarized leases, related-party loans, cross-border operations — the exchange-rate clause, the treatment of fluctuation under ISR and the documentation of the rate applied are designed together. Strategium drafts and puts that entire front in order: contract, invoice, accounting and defense.
Frequently asked questions
Can I agree in my contract on an exchange rate other than the official one?
Yes — the rule in art. 8 of the Monetary Law operates as the debtor's right to discharge in pesos and as a default rule; the parties may agree on specific conversion mechanisms, and in sophisticated contracts they always do. What you don't want is silence.
Is today's FIX the one published today in the DOF?
No — and this confusion causes daily errors: today's DOF publishes the FIX determined yesterday. That's why the legal rules speak of the rate 'published in the DOF' on a given date: technically you're using the previous business day's FIX. For tax and contractual calculations, the publication date is what governs.
Which exchange rate do I use for my return if I collected in dollars?
For tax purposes, the one from the day before the tax accrues (art. 20 CFF) — in practice, the one published in the DOF applicable to the date the obligation was generated. Your accounting must value with that consistency; the year's exchange-rate workpaper is one of the schedules an audit appreciates most.
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