Overcharged on Discounted Items and Voucher Not Refunded

I bought veggies on the 31/03 on 2 different shops 1 order was around R287 The next order was R100.40 For the R237 order I had r100 voucher instead of them taking r1137 they took tha R237 and a R100 voucher in my account For a 2nd order which was r100.40 they adjusted the amount to R133 which is the original prices of the items yet when I bought those fruits they were in 50% off sale cause I bought them on Tuesday Now they telling me they cannot amend that amount yet they were able to increase the amount I need them to pay my r100 and Amend that r133 to r100.40 as per the order