Is it permissible to replace a flawless commodity after the seller attempted to find a solution for the buyer to make the commodity suitable for him, and after the buyer claimed that it was his right to have the commodity refunded?

As long as the seller ultimately agreed to exchange the item, there is no sin upon the buyer in that, and no consideration for his error in mentioning one of the reasons for returning the item initially. What matters is the seller's consent to the return or cancellation, and this is recommendable, based on the Prophet's (peace be upon him) saying: "Whoever cancels a transaction for a Muslim, Allah will cancel his stumble."