All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
It seems – and Allaah knows best – that you are not required to inform the seller that you have found the tool as opposed to what you thought it to be. Also, you are not required to pay him the value of the second-hand motor because the seller might have purchased it as defective and then sold it to you like that, following the custom of second–hand and commodity vendors. He would not be harmed financially in order to demand compensation from you. Most scholars of Fiqh (jurisprudence) have stated that if a seller thinks that the thing he wants to sell is defective yet found it to be good, then the sale should be executed and nothing should be returned to him. This is because the sale has been conducted and all of its conditions and pillars have been fulfilled. As such, the ownership of the commodity has been transferred from the seller to the buyer, and the ownership of the revenue from the sale price has been transferred from the buyer to the seller.
Allaah Knows best.