Disgusting service for exchanging

In February I purchased the huggies 2 jumbo packets for R369. I went to the Babies r us at The Glen to go and exchange the size as the one I had bought was big for my son. I was still within their 60 days return or exchange period. Went and took the correct size and the lady at the till informed me that I needed to pay an extra R40. I refused and asked her to get someone who can assist. The manager also gave me the same story and that their system won’t allow a price change and that there’s another special running.she wanted to charge me for the current special which is very wrong because no where on their slip does it say that if you purchased an item during a special and exchange it after the special is over you will be charged as per the current special. I refused to pay extra and requested for a full refund. Never again will I ever shop there and I will definitely make sure that none of my family and friends shop there too. Appalling service.