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A press enters Mexico in 2021 under a temporary import, runs three shifts a day for five years, receives a new drive, a rebuilt hydraulic pack and a control cabinet that appears nowhere on the original invoice, and then the program ends and the machine goes home. The entry pedimento describes a machine that no longer exists in that configuration. Someone now has to write a number on the return.
Most operations reach for the wrong shelf. They ask finance for the net book value, because that is the number the company has for that asset. It is wrong twice. It is wrong because book depreciation answers an income tax question rather than a customs one, and it is wrong because a return is an export operation, and Mexican law does not value exports the way it values imports.
The export duty on machinery is zero, so nothing bounces at the gate. That is precisely why the number drifts. What it does affect is the discharge of the entry that has been open for five years, and the file that has to survive the review that reads both ends of that operation together.
A return is an export, and exports have no customs value
Articles 64 through 78 of the Ley Aduanera build customs value for goods entering Mexico: transaction value first, then five secondary methods applied in a fixed order, successively and by exclusion. That architecture is the subject of its own piece and this one does not re-teach it. What matters here is its scope. The six methods exist to determine the taxable base of the import duty. They do not reach goods leaving the country.
Article 79 governs the other direction, and it is one paragraph long where the import section runs fifteen articles. The taxable base of the export duty is the commercial value of the goods at the place of sale, stated in the comprobante fiscal digital or equivalent document and, failing that, in any other commercial document, without freight and insurance. There is no hierarchy of methods, no statutory additions, no fallback ladder, and no concept of customs value at all on the export side.
That reads like freedom. It is not. The second paragraph of the same article gives the authority the power to verify and to sanction when it has elements to presume that the values stated are not the commercial values of the goods. The standard is commercial value, and nothing in the law shifts the burden of demonstrating that your figure is one.
On paper, an export line with a zero duty rate has nothing at stake. In practice, that line is the one the authority reads back to you when it reconciles five years of temporary imports against five years of returns, and a figure nobody can explain is a finding regardless of the duty it did or did not generate.
Book value answers a different question
Net book value is acquisition cost less accumulated depreciation, calculated at the rates and over the useful life that the tax and accounting frameworks establish. It exists to allocate the cost of an investment across the periods that benefit from it. It was never designed to say what a machine is worth to anyone.
Two identical presses of the same age, in the same condition, sitting on the same plant floor, can carry different book values because they were acquired on different dates under different policies. A declaration cannot run on a number that moves with an accounting decision.
Where the law does have to put a value on a temporarily imported fixed asset, it anchors to the entry. Rule 1.6.10 of the Reglas Generales de Comercio Exterior for 2026 covers the case where an IMMEX company changes the regime of a fixed asset from temporary to permanent import: the contributions are determined on the customs value declared on the temporary import pedimento. Not a depreciated value. Not a fresh appraisal. The number that was declared on the way in.
That is the principle worth carrying across to the return. In the temporary import world, the entry record is the anchor, and any figure that departs from it has to be explained by something documented that happened in between.
There is a second reason to work from the entry rather than around it. That figure was itself built under Articles 64 and 65, and what it did or did not include was decided by the commercial term agreed with the supplier. Carrying it forward carries a construction someone already defended. Replacing it with a book figure discards that work and starts the argument from zero.
Common mistake. Finance sends the net book value, the customs file adopts it, and no document connects the two numbers. Depreciation can legitimately support a valuation, inside a condition assessment or a public broker's appraisal, when a commercial value genuinely has to be built. It supports the number. It is not the number.
What the return has to reconcile with
The value on the return does not stand alone. It closes a loop that opened when the asset entered, and the mechanics of that closure are explicit.
The return pedimento carries a discharge block. Under Anexo 22 of the RGCE, the company transmits the number, the date and the clave of the temporary import pedimento being discharged, together with the classification and the quantity of the goods being returned. The entry and the exit are joined in the declaration itself, not in a spreadsheet somewhere.
The clave carries the same logic. Fixed assets brought in by an IMMEX company under Article 108, fracción III of the Ley Aduanera enter under AF. The return goes out under RT when it happens under an IMMEX program, and under H1 when the same-state return happens outside one. Two keys in two different families of Apéndice 2, for an operation the warehouse describes with the same word. The clave is not a label chosen at the end; it is the statement of what the operation legally is.
Article 36-A adds the identification layer. For merchandise that was temporarily imported and returns in the same state, where the goods can be individually identified, the serial, part, brand and model data travel with the value information transmitted for the operation. And Article 59, fracción I requires the automated inventory control system, Anexo 24, to show the asset in and the asset out, with Anexo 31 running alongside it for companies certified in the VAT and excise tax modality.
Read together, these say one thing. The number on the return is read next to the number on the entry, by a system built to compare them. A value with no documented path from one to the other is the finding, and it is found years later.
Repairs, upgrades and parts that were swapped
Five years of production leaves a trail on any machine, and the treatments diverge sharply depending on what kind of change it was.
Restoration to original function. Bearings, seals, consumables, a rebuilt motor using parts already accounted for. The asset that leaves is the asset that entered. The entry value holds, and the work orders document why the machine looks different from its 2021 photographs.
Foreign parts brought in separately. Each of those parts arrived under its own pedimento, with its own clock and its own line in the inventory system. When the part leaves inside the machine, it has to discharge its own entry. A machine's return does not absorb the open entries of everything bolted to it.
Mexican components incorporated. A control cabinet built in Monterrey and installed on the press is Mexican merchandise leaving the country. It never had a temporary import to discharge, its value was never part of the original entry, and it does not belong inside the discharge of that entry. It leaves on its own terms.
Parts removed and left behind. This is the one that surfaces last and costs most. A temporarily imported component that comes off the machine and stays in Mexico still has an open entry against it. It needs a destination of its own: a change of customs regime with the corresponding payment, a transfer to another IMMEX company, or destruction under the procedure the rules establish. The machine crossing the border does not close that line. The auditor who finds it open is usually looking at a machine that has been in Ohio for two years.
When the machine that leaves is not the machine that arrived
Temporary imports under Article 106 rest on the premise that the goods return abroad in the same state. Fixed assets under Article 108, fracción III rest on the premise that identifiable machinery serves the authorized program and returns when the program ends.
The identity test is documentary before it is physical. The plate data declared on the return has to trace back to the plate data declared on the entry. When it does not, the return stops being a discharge and becomes a question.
Two consequences follow. First, when the work performed in Mexico was deep enough to produce a different article rather than the same machine improved, the return as the same asset is not the available route, and that determination has to be made before the equipment is loaded, not at the crossing. Second, the tariff classification declared on the return describes the machine as it leaves. If an upgrade moved the classification, the mismatch against the entry is not automatically an error, but it will not reconcile on its own and it has to be explained inside the file.
The other side of the border reads a different number
If the asset goes back to the United States, the entry there is built on a different question. Not what the machine is worth, but what happened to it while it was away. Three situations cover most returns.
Returned unchanged. HTSUS subheading 9801.00.10 covers products of the United States returned after export, and other products returned within three years, provided they were not advanced in value or improved in condition abroad. It is duty-free, and "not advanced in value" is the entire test. Five years of production work and a rebuild do not pass it quietly. CBP has published specific guidance on the documents it may request to support these claims.
Repaired or altered in Mexico. Subheadings 9802.00.40 and 9802.00.50 apply, and under USMCA the treatment is generous: goods returned after repair or alteration in Mexico are eligible for duty-free entry regardless of whether the repair could have been performed in the United States, regardless of whether it increased the value of the good, and regardless of the origin of the good, under 19 CFR 182.112. Duty-free is not value-free. The cost or value of the repair is still declared, and the two declarations required by 19 CFR 10.8, one from the party that performed the work and one from the owner or importer, still go in the file.
Work that produced a new or commercially different article. Then it was not a repair or an alteration, the Chapter 98 route closes, and the entry is a regular import at full value with the origin analysis that follows.
Across more than 190,000 customs operations a year at 39 ports on both sides of the border, the returns that clear without a question are the ones where the Mexican exit file and the United States entry file were built by people who had read each other's work. They describe the same machine with two different numbers, under two different documentary theories, and the reconciliation that matters is the one between them.
Build the record on the day it arrives
What makes this question hard is almost never the law. It is that the answer gets reconstructed five years later by people who were not there when the asset landed.
The file that turns the return into a lookup rather than an archaeology project starts on the day of entry and holds three things. The entry package: pedimento number, clave, declared value, the invoice or document that supported it, the serial and plate data as declared, and the authorization the asset entered under. The change log, maintained per asset for as long as it stays: every repair with its work order and invoice, every foreign part installed with its own pedimento reference, every Mexican component added with its supplier invoice, every part removed and where it went. And the valuation support, added only if a commercial value genuinely had to be built rather than carried forward.
Quick check. Pick one machine that entered temporarily more than three years ago. Ask for the entry pedimento number, the declared value, and the complete list of parts added and removed since. If assembling that takes more than a day, the return will take considerably longer.
The machine that leaves is not the machine that arrived. That is a fact about the equipment, and it is fine. The work is making it a documented fact rather than a discovered one, because the number on the return is only as defensible as the record standing behind it.
. . .
When did your team last pull the entry file on a machine that has been sitting in Mexico for more than three years?
TRADE. UNDER CONTROL.
