What is the ruling of Islamic law regarding planting a mango tree in land owned by one's father, if it later becomes clear that the sapling for planting was taken from endowed land, knowing that the value of the sapling is 2500 Riyals?

It is not permissible to take the aforementioned tree or any other item from the endowment (waqf) except with the permission of the supervisor (nazir). The supervisor is not permitted to dispose of anything unless it serves the interest of the endowment. If the buyer obtained the sapling through an unlawful means, your father must return the sapling to the responsible party if possible; otherwise, he must return its equivalent or its value, because the father's ownership has not been established. Then, the father should demand from the seller what was taken from him, as it was not his right.