Is there any impropriety in the transaction that took place to replace the non-working CD after a period exceeding two weeks, especially since the shop owner agreed to the replacement based on the son's statement that the period was two weeks, and he did not pay attention to the father's statement that the period exceeded two weeks? And is the father's clarification to the employee – after the shop owner had left – that the period might exceed two months, sufficient to remove any impropriety?

It is permissible to return a defective product to the seller and either get a refund or a replacement, as long as the seller agrees to it. The length of time during which the defect was not apparent does not affect this. This return due to a defect is permissible by mutual consent. As long as the seller believes your claim of defect, there is no sin upon you. Ibn Qudamah stated in Al-Mughni that the buyer has the option to either keep or cancel the sale if they discover a defect in the purchased item that they were unaware of, whether the seller knew of the defect or not. A Muslim's sale implies freedom from defects. Therefore, there is no sin in benefiting from the replacement disc provided by the seller.