The first question of Tosfos is how can Rashi understand that when the Gemara says "Lo Chaser" it means that the Makif would have had to put walls up anyway. The only reason the Makif has this expense is because the field of the Nikaf is in the middle of his fields on three sides! The Makif already has outer walls around his fields! Secondly, the Gemara should read that you caused me to make all of these surrounding walls, not merely "an extra amount of surrounding (walls)!"

Tosfos therefore explains that the case is not as stated by Rashi, that walls already surround the Makif's field from the outside, and he is now putting up walls on the inside between himself and the Nikaf. Rather the case is where he is only putting up walls outside his field. This he would do regardless of the Nikaf's field being in the middle of his three fields. However, if the Makif's fields were side by side, he would have to put up a much shorter wall on the fourth side. The fact that he has to put up a long wall is due to the presence of the field of the Nikaf.

All the best,

Yaakov Montrose