What is the ruling on mistakenly substituting one owned item for another, if my owned item was defective and I do not know its owner, and does this necessitate giving it in charity on behalf of its owner or disposing of it? And what is the ruling if the two exchanged items are identical and neither has a defect?

As long as you are the one who erred by taking something that is not yours and leaving what is yours, then you do not own it. You must search for its owner, and if you do not find him after a year, then give it in charity on his behalf. And whoever finds something that is not his instead of something that belongs to him, he must also search for its owner and make it known as lost property (luqatah). If he does not find him, he may take from it an amount equal to his right and give the rest in charity, or he may hand over the excess to the ruler.