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A payee's employment/payslip details are defined via the Portal > Pay > Payee Payroll > Payee Maintenance > Payee Entry > Employment/Payslip Details section.
Once defined, a payee's employment/payslip details can be modified if necessary so long as the payee is not currently included in an open pay batch.
However, once the payee has been paid (included in a closed pay batch) their employment/payslip details are locked down for the corresponding payee details validity period and cannot be modified for that period. If you need to modify a payee's employment/payslip details after the payee has been paid in one or more pay batches, you must create a new payee details validity period.
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