Is the landowner's removal of the structure that his neighbor unjustly erected on his land considered an act of oppression, especially since the neighbor claims that the original owner promised them this part of the land, even though the documents prove the landowner's ownership of it?

If your neighbor encroaches upon your land and builds on it, you are permitted to demolish his structure and remove it. This is not considered an act of injustice, nor are you obligated to forgive him. In such a case, if he invokes a curse upon you, he is an oppressor, and his supplication will not be answered because it contains sin.