Are we entitled to keep the money we took from the landowner in exchange for the trees that my grandmother planted on his land, knowing that my grandmother cultivated the land without documentation and the ownership of the land belongs to the man?

If the landowner pays the value of the trees willingly, then there is no objection to acquiring the money from your grandmother's estate. However, if he is coerced, then it is not permissible, because he has the right to demand the uprooting of the trees and the leveling of the land.