Ackermans refunds sale price and not what was paid

My sister-in-law took out a lay-bye for her kids. She collected it on the 26/07/2018. Her boys eventually tried on the clothes, and it was too big.... She took it back on the 15/08, with the invoice and tags still attached, just to find out the items were marked to half price. The store then refunded her the sale price, and not the price she paid.... Yes when she collected the lay-bye the items were already marked down but because it was in a lay-bye she paid the full price. This makes absolutely no sense, and is completely ********* to do so.