A man entered the state of ihram for Hajj and wore trousers under the izār out of ignorance; what is required of him?

Question : A man entered the state of ihram for Hajj. He kept his trousers on. He wore the izār over the trousers. He did not know that wearing trousers is forbidden. What is required of him?

Answer : He owes nothing at all. But once he knows, he takes off the trousers.