A payroll integration is a promise that data will arrive somewhere else correctly and on time. Most of them work on ordinary weeks and fail on the unusual ones, which is where the testing effort belongs.
Know which direction each field travels
Hours and approved exceptions usually flow into payroll. New starters, leavers and pay changes may flow either way depending on which system is the master. Write that down field by field before anything is configured, because the commonest cause of contradictory records is two systems both believing they own the pay rate and each overwriting the other on a different schedule.
The four failures worth designing against
A record rejected and nobody told; a partial file loaded as if complete; a duplicate run doubling someone's hours; and a mid-period change arriving after the cutoff. Each needs an answer before go-live: an error report that reaches a person, a record count reconciled at both ends, an idempotency check, and a written rule for late changes.
Test the awkward cases, not the happy path
A mid-period pay rise, a leaver paid in their final period, a correction to an approved timesheet, a rehire reusing an old identifier, and somebody with two roles at two rates. Those five cases surface nearly every design flaw, and they are much cheaper to find in a parallel run than in a live pay period with people waiting.
Run one period in parallel before you trust it
Produce the payroll both ways for one full period and reconcile to the cent, by person. Differences are either a real defect or a rule you did not know existed, and both are worth finding then. Skipping parallel running is the decision most often regretted, because the alternative discovery route is an employee noticing their payslip is wrong.
Questions people ask about payroll integration
Is a file export an integration?
It counts if somebody owns the schedule, the error handling and the reconciliation. An unowned file drop is a hazard with a filename.
Who should own the integration?
One named person, usually in payroll rather than IT, because the failures are detected in payroll and have a deadline.
How often do these break?
Most often after an upgrade at either end, which is why a scheduled reconciliation matters more than a one-time test.