Expatriate payroll is the machinery of paying an employee who works outside the country their employer sits in, without breaking either country's tax and social security rules. For a US employer the starting point is uncomfortable: US federal income tax withholding generally follows a US citizen's wages abroad, and social security obligations can arise in two countries at once unless a treaty says otherwise. The tooling that manages this, exemption forms, totalization certificates, shadow payroll, is well established but paperwork-heavy, which is why long assignments are often restructured onto local employment or an employer of record instead.
US withholding follows the employee abroad
The IRS's rule for persons employed abroad by a US person is that wages paid to a US citizen or resident for services performed outside the United States are generally subject to US federal income tax withholding, with limited exceptions. The main relief valve is the foreign earned income exclusion: an employer can stop withholding where it is reasonable to believe the pay will be excluded under section 911, which the employee claims by filing Form 673. The IRS is specific about one trap: a resident alien may not use Form 673 to exempt wages from withholding, even if they expect to qualify for the exclusion when they file. So the same assignment can carry different payroll treatment for a citizen and a green-card holder sitting side by side.
Social security: totalization and the detached worker rule
Without a treaty, an expatriate can owe social security contributions in both the home and host country on the same wages. The United States has bilateral totalization agreements with 28 countries designed to eliminate that dual taxation and to fill benefit gaps for careers split across borders. The default principle is territorial: the worker is covered where the work happens. The exception that matters for assignments is the detached worker rule: an employee sent abroad by a US employer can stay under US coverage when the assignment is expected to last five years or less, documented by a certificate of coverage presented to the host authorities. Italy's agreement is the notable outlier, lacking the standard detached worker rule and turning largely on nationality instead.
Shadow payroll and split payroll: the mechanics
Where a host country taxes the assignee's income but the salary keeps being paid from home, employers run a shadow payroll: a parallel host-country payroll that reports the compensation and remits local withholding and contributions without actually paying the net salary twice. Split payroll divides the actual payment between home and host, often to manage currency needs and local benefit bases. Both require someone in the host country entitled to operate a payroll, which usually means a local entity, a registered branch or a provider acting for one. Assignment policies also commonly add tax equalisation, keeping the employee's personal tax burden at the home-country level while the employer absorbs the difference; that is a policy choice, defined in the assignment letter, and the documents control.
When an EOR replaces expatriate payroll
Classic expatriate payroll assumes a temporary assignment with a home employment relationship to return to. When the move is really permanent, or the company has no entity in the host country at all, restructuring onto local employment through an employer of record is often simpler: the EOR employs the person under host-country law, runs local payroll and contributions natively, and the dual-country machinery largely falls away, leaving the individual's own filing obligations. The trade-offs are real: home-country benefit accrual and social security continuity can be interrupted, which is exactly what totalization certificates preserve on a truly temporary posting. The right structure follows the facts of the assignment; none of this is legal or tax advice, and cross-border cases justify specialist review.
Questions people ask about expatriate payroll
Does US withholding stop when an employee moves abroad?
Not by default. Wages a US person pays a US citizen or resident for work abroad generally remain subject to withholding; it stops for a citizen only where the foreign earned income exclusion is reasonably expected and Form 673 is in place, and resident aliens cannot use Form 673.
Can an employee owe social security in two countries at once?
Yes, where no totalization agreement applies. The US has agreements with 28 countries; under them a worker sent abroad for an expected five years or less can generally remain under US coverage with a certificate of coverage.
What is a shadow payroll?
A host-country payroll run purely to report compensation and remit local withholding while the salary is actually paid from the home country. It satisfies local obligations without paying the employee twice.
When is an EOR better than an expatriate arrangement?
When the relocation is effectively permanent or there is no host-country entity. The EOR employs the person locally and the dual-payroll machinery falls away; for genuinely temporary postings, totalization coverage and home benefits often argue for keeping the assignment structure.