Is exchanging new bread for old after leaving the store considered a usurious sale, or another type of sale such as a mu'atah (transaction by mutual consent) sale?

There is no harm in replacing purchased bread with another loaf, because returning the first is a cancellation (of the sale), and buying the second is with the price of the first. Some scholars have permitted exchanging bread for bread of the same type under certain conditions, such as estimating the flour content or weighing it if it is of the same kind, especially with the current precision in bread making regarding its weight and moisture.