Is it obligatory to return the remaining amount from the computer's price to the seller and inform him of the truth, or is it permissible to donate it on his behalf, knowing that not informing him of the matter causes embarrassment to the questioner?

What you mentioned, that you discovered the seller changed the agreed-upon specifications, gives you the option to return the sold item. However, your retention of the device and not returning it indicates your acceptance of it, and therefore you are obligated to pay the entire agreed-upon price. Thus, you must repent to Allah for refusing to pay the remaining amount by returning it to its owner, and it is not sufficient to give it as charity on his behalf. And Allah knows best.