Is buying merchandise for the same amount stolen from a store and then returning it to get the money back, or donating the amount with the intention of it reaching its original owner, valid for clearing one's conscience?

There is no sin for what you stole before puberty, according to the hadith: "The pen has been lifted from three: from the insane person whose mind is overcome until he recovers, from the sleeper until he awakens, and from the child until he reaches puberty." However, you must guarantee what you stole. Paying the value of the items to the shop and then giving the same amount in charity is sufficient. If it reaches the shop owner, then the objective is achieved. If it does not reach him and he is unknown, then giving the amount in charity on his behalf is required.