Just checked UK law. If you receive an extra item as part of a replacement for a broken item or similar and use it the sender has the right to demand payment for the extra item. The correct course of action is to contact the sender who can then choose to collect at their own expense.
I was surprised by this - I thought it would constitute unsolicited goods but it does not due to the extra item having been sent as remedy for a broken item. If someone sends you something out of the blue and then demands payment it is unsolicited goods and you have the right to keep it.