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).
Selling your company's shares — to a partner, to a third party, to the next generation — is the transaction where the most money is won or lost through tax technique, for one brutal reason: the default regime withholds 20% on the total PRICE, while the well-executed regime pays progressive rates on the real GAIN computed with your adjusted tax basis. The difference between the two can run into the millions. Here's the full map: corporate, tax, costs and timelines.
The corporate mechanics: more than signing a contract
A valid transfer requires the full chain: a sale/assignment contract (with the price, payment traceable through the banking system, and representations), respect for the rights of first refusal and bylaw locks (the partners' preemptive right, meeting authorizations — selling to a third party while jumping the preemptive right is an invitation to nullity), an entry in the share registry ledger (as against the company and third parties, the owner is whoever is recorded — the neglected ledger is the classic corporate time bomb), endorsement of the certificates if they exist, and the notices: updating the ownership structure in the RFC (the federal taxpayer registry) and the controlling beneficiary registry. In an S. de R.L., the transfer of membership interests adds its own formalities (partner consent per the bylaws and notarization). None of this is decorative: the corporate file IS part of the transaction's tax defense — and with reliable date (fecha cierta), of course.
The heart of the tax question: 20% of the price vs. progressive rates on the gain
Default rule for the individual seller: the buyer withholds 20% of the total price as a provisional payment (art. 126 LISR) — on the price, not the gain: you sell shares that cost you $8M for MXN $10M, and the withholding is $2M against a real gain of $2M. The law's escape valve: the seller may elect a lower withholding computed on the real gain when the transaction is accompanied by a public accountant's tax opinion (dictamen) (registered, with the calculation working papers) — the procedure that turns 20% of the price into progressive rates on actual profit, and the reason no serious share sale is signed without its dictamen. Computing the gain is the house specialty: price minus average cost per share — proven acquisition cost adjusted by the CUFIN balances (the profits already taxed at the company raise your cost: they're not taxed twice) and the movements in CUCA, capital reimbursements and losses, all restated for inflation (art. 22). A company with years of retained earnings can have an enormous tax basis — and the seller who fails to compute it gives away exactly that difference. The annual return closes the cycle: the gain is accumulated under its softened mechanics (part at the progressive schedule, part at the effective rate for the years of holding) and the withholdings are credited.
Costs, timelines and the variants that change the game
Market costs (estimates): a disposal tax opinion, $50–150k depending on complexity; corporate/notarial, $20–80k; the buyer's due diligence, separately. Timelines: 4–10 weeks done properly — the tax opinion and the reconstruction of CUFIN/CUCA set the pace (companies with disorderly historical accounting: more, and it's the seller who pays for that disorder in tax basis that can't be proven). The variants, each with its label: a sale at fair value with a tax opinion — safe, it's the law's design; a sale between related parties (family, your holding) — defensible with an appraisal/valuation as support: a price freely agreed between related parties without support is an adjustment served up on a plate (and the "cheap" sale to a child also reads as a gift with its own rules — which is sometimes exactly the right vehicle, used openly); and the sale with a simulated or eternally deferred price to "avoid triggering it" — red zone: the authority recharacterizes with the bank file in hand. For wealth transfers to the next generation, always compare against the alternatives: a gift of shares in a direct line (exempt with formalities) and a trust (fideicomiso) — selling is not always the right vehicle for what you actually want to do.
About to sell, buy or reshuffle shares — even 'within the family'?
The prep work decides the bill: reconstructing CUFIN/CUCA and the tax basis per share (we frequently find unused basis worth millions), the tax opinion to withhold on the real gain, valuation support for related-party transactions, and the comparison against a gift or a trust if the underlying goal is succession. A share restructuring without this analysis is signing the biggest check of the deal blind.
Frequently asked questions
What if the buyer is an individual or a foreigner — who withholds?
The mechanics change, not the obligation: between individuals, the seller remits the 20% provisional payment on the price (same escape valve via the tax opinion); a foreign buyer or a foreign seller triggers the regimes for non-residents with their withholdings and, where applicable, treaty benefits — each combination has its own route. The universal constant: without the tax opinion, the system treats you under the gross rule.
I sold shares in my company and 'nothing happened' three years ago — is that a problem?
It depends on what didn't happen: if there was no withholding/remittance and no reporting of the gain, you have an omission with surcharges accruing (and the transaction is visible: the ledger, the shareholders' RFC, the money flows, the controlling beneficiary). Voluntary regularization is still the cheap way out — and the 5-year statute of limitations won't protect you if the file has the aggravating factors that extend it.
Does 'selling at par value' work to avoid paying?
It's the classic that doesn't work: par value is not market value, and a sale notoriously below value triggers presumptive-assessment powers (via appraisal) plus a reading of a disguised gift to the buyer. If the real goal is to transfer cheaply to family, the correct and exempt vehicles exist — using them openly costs less than disguising.
What's this about the gain being 'divided across years of holding'?
It's the individual's accumulation mechanics: the gain is split — one portion goes to your rate for the year and the rest pays at the resulting effective rate, multiplied by the years of holding (capped at 20). The practical effect: long holdings soften the progressivity — one more reason why timing and the documented history of your shares are worth money.
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