Is a lawyer's taking 40% of the value of recovered land as his fee considered "a loan that drew benefit"?

This is a ju'alah (a contract of reward), not a loan that brings benefit. It is a condition of a ju'al that it be known and specified, based on the Prophet, peace and blessings be upon him, saying: "If you hire a laborer, inform him of his wage." So, if the land due to the mother is known in terms of its area and location, there is no objection to the contract. However, it is preferable to specify a fixed fee for the lawyer to avoid disagreement.