Is it permissible to benefit from spare phone parts left over after maintenance, especially if they are usable and you forgot to return them to the customer?

It is not permissible to take or keep anything without the owner's permission, even if it is something spoiled that can still be used. It must be returned to the owner unless the owner permits it or it is customarily overlooked. This is based on the Prophet's (peace be upon him) sayings: "The wealth of a Muslim person is not lawful except with his willing consent," and "The hand is responsible for what it takes until it returns it."