Can an employer simply change the allowance hours and allowance amount?

Questioner

I work for a retail company. My employment contract states that there is a 200% surcharge on Sundays and a surcharge of 125% in the evenings from 6-9 pm. My employer has now just announced that the evening surcharges will be discontinued and the Sunday surcharge will be 150%. Are they allowed to do this? They state that this is determined by law, but I cannot find anything about this myself.

Lawyer

If you have agreed on certain allowances, the employer may not simply change this agreement unilaterally. Employment conditions can be changed unilaterally under certain conditions, but very specific conditions apply to this.