Add the medical aid contribution as a deduction, then add a negative non-IRP5 (XXXX) deduction for the same amount to balance it out. You are welcome to create a pay transaction template "Self-paid medical deduction" and add it as a pay rate under the employee profile. The negative deduction will balance out the employee self-paid medical contribution.

Changing the medical aid member quantity on the employee profile; and editing and saving the payslip again will not change a medical aid tax credit already generated for that payslip. It will only affect future payslips. This is to provision for manual overrides in partial period payslips. You can however manually update the medical aid tax credit by viewing the payslip and editing the medical aid tax credit amount.