How can I add an (after or less) payment of a former period or and advance payment in a pay slip?
In fact the salary owed to this employee has already been processed and taxed earlier, so you just want to add the net amount to the current pay slip. Also advance payments that have to be paid are untaxed.
The best way to do this is duplicate wage code 410 and name it the name you want, enter the amount owed or advance payment in this code and finalize the pay run.
Wage code 410 is an untaxed wage code that will not end up in 'untaxed allowances'' in the annual Wage Tax Card (in Dutch: Jaaropgaaf, Loonbelastingkaart and Loonopgaaf).