Is selling surplus wood after unpacking goods and receiving payment for it, knowing that it used to be paid for in the past to dispose of it, considered haram, and is it necessary to inform the administration of this?

If the owners of the establishment do not want the wood and order it to be thrown away, there is no harm in taking it and benefiting from it without informing them; because it is an implicit permission from them and an indication that they have relinquished their claim to it. In fact, it is better to take it and sell it, because throwing it away is extravagance and a waste of money, and Islamic law has prohibited that.