Bad service and unable to redeem promotional goods

I was at the Bloem branch today. was told by the consultant that they are running a promotion to LYB members - if you purchase for R350 or more you get R100 off. so i got my products and when i paid. i used my Birthday voucher and the consultant informed me that i can not pay with my voucher because of the promotion unless my net amount after using the voucher must still be R350. i do not understand why because no where is it stated that you can not use voucher to pay. an no where was it stated that the voucher can not be used wth other promotion. consultant was very rude and unwilling to assist. can u plz confirm if this was handled correctly?