פירוש על קידושין 117:3

The Talmud asks why our chapter calls the one sent to betroth, “his neighbor” while the other chapter calls him “his agent.” It suggests that in both cases the mishnah uses a word that is “new” meaning it teaches us something that we would not have otherwise thought.
Here it teaches “neighbor” to let us know that even though one does not generally rely on neighbors for such types of favors, even so, the neighbor who betroths the woman to himself is still considered deceptive.