Is retrieving the price after agreeing with the seller to return the merchandise, then later discovering a sign that says "Sold goods are not refundable or exchangeable," considered a wrong action that necessitates expiation?

If the seller agrees to the buyer's condition of returning the clothes, then there is no blame upon the buyer for returning them. This is because a conditional option (khiyar al-shart) is permissible if both parties agree to it. The presence of a sign saying "Goods sold are not returnable or exchangeable" does not change anything if the seller explicitly agrees to the condition. If the duration of the option is not agreed upon, then Imam Malik's madhhab is preferred, which permits it and determines its duration according to custom.