Spoiled cake

My brother bought a cake from Groblersdal Spar for my grannies birthday on the 4th of February but when we cut the cake and served guests on Sunday the 5th the cake tasted as though it is very old besides the label written that the expiry date is on Monday the 6th. We have cut and served people when everyone started complaining about the smell and taste. We had to throw the cut pieces and there were extra still left that we didn't serve anyone. My sister took the left slices and slip to Spar however she was not refunded nor given a new fresh cake but they took the slices and refused to give them back saying that they cannot refund or exchange because the cake has been cut but how would we have known that the cake was not good if we never even ate a piece of it.