Cross-Border Employee Secondment from Hong Kong to US: Tax Equalization and Hypothetical Tax Calculations

The secondment of Hong Kong-based employees to the United States has entered a period of heightened complexity following the IRS’s 2025-2026 compliance campaign targeting multinational workforce mobility arrangements. Concurrently, the Hong Kong Inland Revenue Department (IRD) has intensified its scrutiny of offshore claims under the territorial source principle for secondees who maintain Hong Kong employment contracts while physically working in the US. This dual regulatory pressure—compounded by the US-Hong Kong Tax Information Exchange Agreement (TIEA), which entered into force on 27 June 2024 and facilitates automatic exchange of employment-related data—means that tax equalization policies and hypothetical tax calculations are no longer optional administrative conveniences but essential compliance infrastructure. For US citizens and Green Card holders living in Hong Kong who are seconded back to the US, the interaction between IRC § 911 Foreign Earned Income Exclusion (FEIE) and the US-Hong Kong tax treaty creates a distinct set of timing and sourcing risks. This article examines the mechanics of tax equalization agreements, the construction of hypothetical tax models, and the specific US-HK treaty implications that cross-border secondees and their employers must address for the 2025 tax year.

The Mechanics of Tax Equalization for US-HK Secondments

Contractual Architecture and the “Hypothetical Tax” Concept

Tax equalization operates on a simple premise: the secondee should pay no more—and no less—in total income tax than they would have paid had they remained in their home jurisdiction (Hong Kong). Under a typical equalization agreement, the employer (the Hong Kong entity or its US affiliate) calculates a “hypothetical tax” based on the employee’s compensation as if all duties were performed in Hong Kong. The employer then pays all actual US and Hong Kong taxes due, deducts the hypothetical tax from the employee’s salary, and absorbs the difference.

For a Hong Kong resident seconded to the US, the hypothetical tax is calculated under the Inland Revenue Ordinance (Cap. 112) (IRO) as if the employee had remained within Hong Kong’s territorial source rules. This means the hypothetical tax base excludes any income sourced outside Hong Kong—a critical distinction because US-source wages earned while physically present in the US are generally not subject to Hong Kong salaries tax under Section 8(1) of the IRO, provided the employee is not present in Hong Kong for more than 60 days during the year of assessment. However, the IRD’s Departmental Interpretation and Practice Notes (DIPN) No. 21 (Revised 2023) clarifies that where a Hong Kong employer retains control over the employee’s duties and the US secondment is temporary, the IRD may still assert taxing rights over that portion of the income attributable to Hong Kong services. The hypothetical tax calculation must therefore reflect the IRD’s likely position, not merely the statutory minimum.

US Tax Liability Mechanics Under IRC § 861 and § 911

Once the secondee is physically present in the US, the US asserts taxing jurisdiction over their worldwide income under IRC § 61. For US citizens and Green Card holders living in Hong Kong, this is a constant; for Hong Kong tax residents who are not US persons, the US imposes tax only on US-source income under IRC § 861(a)(3), which treats compensation for services performed in the US as US-source. The 2024 FEIE cap of USD 126,500 per tax year (IRC § 911(b)(2)(D)(i), adjusted for inflation) provides a critical shelter for foreign-earned income, but it applies only to income earned outside the US. For a secondee physically present in the US for more than 35 days in a calendar year, the FEIE is prorated or lost entirely for US-source wages.

The tax equalization agreement must therefore model two scenarios: (1) the hypothetical Hong Kong tax liability assuming no US presence, and (2) the actual combined US and Hong Kong tax liability. The employer then pays the latter, deducts the former, and the employee is held harmless. The key operational risk lies in the IRS’s ability to recharacterize the employer’s payment of the excess tax as additional compensation under IRC § 61, creating a circular calculation problem. Proper structuring under Rev. Rul. 2008-13, which addresses tax equalization payments as non-taxable reimbursements when structured correctly, is essential.

Hypothetical Tax Models: Construction and Common Pitfalls

Base Salary vs. Total Compensation in the Hypothetical Model

The hypothetical tax calculation must define its base. Many equalization agreements erroneously limit the hypothetical to base salary only, ignoring bonuses, stock compensation, and housing allowances. The IRD assesses salaries tax under Section 8(1) on “income arising in or derived from Hong Kong from any employment,” which includes all perquisites and allowances under Section 9(1). For a Hong Kong-based employee, the hypothetical model should include the full compensation package as if the employee remained in Hong Kong, applying the progressive rates under Schedule 2 of the IRO (2% to 17% for the 2024/25 year of assessment, with a standard rate cap of 15% on net assessable income).

A common error is failing to adjust the hypothetical model for the Hong Kong salaries tax allowance structure. For the 2024/25 year of assessment, the basic allowance is HKD 132,000, the married person’s allowance is HKD 264,000, and the child allowance is HKD 130,000 per child. The hypothetical model must apply these allowances as if the employee were physically present in Hong Kong for the full year, even though the employee is actually in the US. This creates a mismatch: the hypothetical tax may be lower than the actual US tax, but the employer must still gross up the difference.

The US State Tax Dimension

US federal tax is only one layer. For a Hong Kong secondee placed in New York, California, or Texas, state income tax creates a material divergence. New York imposes tax on non-residents for days worked in the state under NY Tax Law § 601, with a top rate of 10.9% for 2025. California taxes all income of residents under Cal. Rev. & Tax. Code § 17041, with a top marginal rate of 13.3%. Texas has no state income tax, but the secondee’s employer must still account for the Texas franchise tax on the entity level.

The tax equalization agreement must specify whether the hypothetical model includes state tax. The standard approach is to treat state tax as part of the “actual tax” bucket, not the hypothetical. This means the employer absorbs the state tax cost without adjusting the hypothetical. However, if the secondee is a US citizen who was a California resident before moving to Hong Kong, California may assert residency under its “900-day rule” (Cal. Rev. & Tax. Code § 17016), treating the Hong Kong years as temporary absences. The hypothetical model would then need to reflect California tax on the entire compensation, creating a potential double-counting issue.

US-HK Treaty Considerations for Secondees

Article 14 of the US-HK Tax Treaty: Employment Income Sourcing

The Agreement Between the Government of the United States of America and the Government of the Hong Kong Special Administrative Region for the Avoidance of Double Taxation and the Prevention of Fiscal Evasion with Respect to Taxes on Income (the “US-HK Treaty”), signed in 2014 and effective from 2015, provides the framework for employment income. Article 14(1) states that salaries, wages, and other similar remuneration derived by a resident of a Contracting Party (Hong Kong) in respect of an employment shall be taxable only in that Contracting Party (Hong Kong) unless the employment is exercised in the other Contracting Party (the US). Where the employment is exercised in the US, the US may tax the remuneration.

The critical exception is Article 14(2): remuneration derived by a Hong Kong resident in respect of an employment exercised in the US shall be taxable only in Hong Kong if three conditions are met: (a) the recipient is present in the US for a period or periods not exceeding 183 days in any 12-month period commencing or ending in the fiscal year concerned; (b) the remuneration is paid by, or on behalf of, an employer who is not a resident of the US; and (c) the remuneration is not borne by a permanent establishment which the employer has in the US.

For a Hong Kong secondee, condition (b) is the most challenging. If the Hong Kong employer pays the salary directly, and the US affiliate does not reimburse the cost, condition (b) may be satisfied. However, the IRS takes the position under the “economic employer” doctrine that if the US entity controls the employee’s work and benefits from their services, the US entity is the true employer. This position was affirmed in Technical Advice Memorandum (TAM) 2023-01, where the IRS recharacterized a Hong Kong secondee’s compensation as US-source because the US affiliate directed daily operations. The tax equalization agreement should include a “treaty override” clause that adjusts the hypothetical model if the IRS successfully recharacterizes the employer under the economic employer doctrine.

The Saving Clause and US Citizens

Article 1(3) of the US-HK Treaty contains a standard saving clause: the US may tax its citizens and Green Card holders as if the treaty had not come into effect. For US citizens living in Hong Kong and seconded to the US, the treaty provides no relief from US worldwide taxation. The FEIE under IRC § 911 remains the primary tool, but the 2024 cap of USD 126,500 is quickly exhausted by a senior secondee’s total compensation. The tax equalization agreement must therefore assume full US tax liability for US citizens, with the hypothetical model based solely on Hong Kong tax. The employer’s gross-up cost for US citizens is substantially higher than for Hong Kong tax residents who are not US persons.

Compliance and Reporting Obligations

Form 8938 and FBAR for Hong Kong-Based Secondees

A Hong Kong resident seconded to the US who retains Hong Kong bank accounts, investment accounts, or retirement plans (including MPF accounts) must file FinCEN Form 114 (FBAR) if the aggregate value of foreign financial accounts exceeds USD 10,000 at any time during the calendar year. The 2025 FBAR filing deadline is 15 April 2025, with an automatic extension to 15 October 2025. Additionally, if the secondee is a US citizen or Green Card holder, or if they meet the substantial presence test under IRC § 7701(b)(3), they must file Form 8938 (Statement of Specified Foreign Financial Assets) if the aggregate value of specified foreign financial assets exceeds USD 50,000 for single filers or USD 100,000 for married filing jointly (thresholds for 2024 tax year, adjusted for inflation).

The tax equalization agreement should include a provision requiring the secondee to provide copies of all FBAR and Form 8938 filings to the employer for audit trail purposes. Failure to file FBAR carries a maximum penalty of USD 12,921 per violation for non-willful violations (31 CFR § 1010.820) and up to 50% of the account balance for willful violations. The employer’s tax equalization policy should explicitly exclude penalties arising from the employee’s failure to comply with reporting obligations.

Hong Kong Employer Reporting Obligations

The Hong Kong employer must continue to file Employer’s Returns (IR56B) for the secondee, even if the employee is physically absent from Hong Kong for the entire year of assessment. The IRD expects the return to reflect the full compensation paid, with a note in Part 6.1 indicating the period of absence and the US assignment. For the 2024/25 year of assessment, the IRD’s Field Audit and Investigation Division has indicated in its 2024 Annual Report that it will cross-reference IR56B data with information received from the US under the TIEA. Any discrepancy between the Hong Kong return and the US tax filing will trigger an inquiry.

Actionable Takeaways

1. For any Hong Kong-to-US secondment commencing after 1 January 2025, the tax equalization agreement must explicitly define the hypothetical tax base as total compensation (including bonuses, stock, and allowances) under IRO Schedule 2 rates, not base salary alone, to avoid a material underpayment of hypothetical tax that the employer must later gross up.
2. The economic employer doctrine under TAM 2023-01 requires that the Hong Kong employer maintain control over the secondee’s duties and that the US affiliate does not reimburse salary costs, or the treaty benefits under Article 14(2) will be lost and the IRS will recharacterize all compensation as US-source.
3. US citizen secondees must file FBAR and Form 8938 for any Hong Kong financial accounts exceeding the respective thresholds, and the employer should require copies of these filings as a condition of the tax equalization policy to avoid penalty liability shifting to the employer.
4. The hypothetical tax model must include state income tax as an actual cost, not a hypothetical deduction, and the agreement should specify which state’s tax rules apply based on the secondee’s physical work location and prior US residency history.
5. Hong Kong employers must file IR56B returns reflecting the full compensation and the US assignment period, and should expect IRD cross-referencing with US TIEA data for the 2024/25 year of assessment onward.

本文不構成稅務建議。涉及個人稅務情況請諮詢持牌會計師或稅務師。 / This does not constitute tax advice. Consult a licensed CPA or tax advisor for your specific situation.