You own the property as it is described in whatever documents were
involved in its sale to you, and no back-door deal can change that.
Perhaps you meant to say “What would be the best course of action be if ur next door neighbor legally leased 200ft of land from ur backyard to build a double fence on his backyard, by going to the land office and for $10 there, u can get a 99 year lease on the land, he spent $20 to get a 198 year lease on the land?”