You should be able to do a buy and sale agreement without any real estate fees. Always a good idea to run it by an attorney but as long as both parties own the land and there aren't any weird deed issues it should be a simple transaction. I would have it surveyed with stakes driven at each corner of the new boundary just to make it clear in the future where the fence line is and who owns what. The current neighbor might be friendly but no guarantee that the next one will be.