An assignment letter with dates and deliverables, a statement of the position kept abroad, the reporting structure during the assignment, and records of how employment continues. Add the receiving entity's plan for the same period so both sides describe one assignment rather than two separate arrangements.
Write the return into the assignment letter
A letter that ends at the departure date leaves the most important part unwritten. State what the person returns to, who they report to on return, and what happens if the project finishes early or late. Keep it factual: a described role with a named owner is evidence, while a promise nobody has authority to make is not.
Have the overseas manager, not only the receiving team, confirm the wording. One further document is worth requesting while the assignment is being negotiated rather than after: written confirmation of who continues to employ and pay the person during it. Where a receiving entity picks up the salary, where a cost is recharged between companies, or where the person moves onto a local payroll partway through, those arrangements affect the description of the employment relationship and are far easier to record at the time than to reconstruct afterwards.
Hypothetical example: a rare-book conservation specialist's assignment is drafted with the receiving institution paying a stipend and the home employer continuing base salary, and setting that out plainly in the assignment letter removes a question that would otherwise be answered by two accounting departments with different recollections.