Is it permissible for me to exchange an item I bought from a store for another item of the same price without the knowledge of the store owner, given that the store's policy allows returns, and do I owe expiation if that is not permissible?

What you are doing—returning merchandise and taking other items without the seller's knowledge—is impermissible. Although you have the right to return merchandise due to a defect or a condition of option, this does not permit you to take anything from his property without his knowledge and permission. You must return what you have taken; if it is no longer available, then its value or an equivalent. You are entitled to take back the merchandise you returned, or its value if the seller does not believe you. The right of option must be for a known period, and the buyer may not benefit from the merchandise during the option period except for testing.