הגה"ה כתב בספרים אבל מקיף לא יהיב ליה אלא דמי רביעית – A gloss . It is written in certain text, ‘However if the מקיף made the fourth wall, the ניקף pays him only the value of the fourth wall’.
In the לישנא בתרא there are two different גירסאות that dispute what position does ר' יוסי maintain, in the case where the מקיף made the fourth wall.
Some texts maintain that ר"י is of the opinion that the ניקף owes nothing if the מקיף built all four walls. This position is seemingly supported by the גמרא in ב"ק which articulates the argument of the ניקף; ‘I only require a נטירא בר זוזא’. This seemingly exempts him from any (additional) payments.
The texts that maintain the even according to ר"י the ניקף must pay for the fourth wall, will argue that even though a נטירא בר זוזא is sufficient, nevertheless the ניקף must pay for the fourth wall. This is based on the assumption that any person is willing to assist in the building of one wall when in return, he is fenced in on all sides.
ואי לא דאיננא לך כרב הונא אליבא דרבי יוסי – And if not, I will pass judgment against you, as per the ruling of ר"ה according to ר"י.
תוספות maintains that רבא is in agreement with the ruling of ר"ה אליבא דר"י. If not, he would not have made this empty threat, but rather would have warned רוניא that there are ways to coerce him to pay רבינא the אגר נטירא.
An empty threat may sometimes be used, when there is no other way to coerce the wrongful party, as in the case of שמואל and the הדסים merchants.
Another indication that רבא agreed with ר"ה אליבא דר"י is the fact that he referred to s'רבינא offer as a compromise, indicating that legally רבינא could have demanded more.
Summary: תוספות offers two interpretations of this story:
A. רבינא boxed in two fields with his surrounding fields. רוניא owned one of this fields, then bought the other. רב ספרא explained using the example of curing hide, that it cost the same amount to cure a large hide as a small hide. Similarly the gain for רוניא is the same as the gain for רבינא.
B. רוניא, who was the sharecropper of רבינא bought a field adjacent to those of רבינא, which would enable him to take care of his own field whilst he is caring of the fields of רבינא. Afterwards רב ספרא corrected רבינא by pointing out that the cost of labor צללא, is the same as the cost of material צלא. This places the worker and the owner on an equal basis as far as בר מצרא is concerned.
מארבע אמות ולמעלה אין מחייבין אותו כו' בחזקת שלא נתן – From four אמות and higher we do not obligate him to pay etc., it is assumed that he did not pay.
The defendant is not believed that he already paid for the wall in a case where בי"ד is aware that he originally refused to participate in building the wall (above ד' אמות). In such a case he does not have the מיגו that he built the wall (himself). It is therefore assumed that the plaintiff built the wall.
אף על פי שלא נתן [עליו את ה]תקרה מגלגלין עליו את הכל – Even though he did not place the beam over the wall, nevertheless we extend his liability for the entire wall.
זה נהנה וזה לא חסר is פטור only if there is no גילו דעת on behalf of the beneficiary; however if the beneficiary is מגלה דעת that he is pleased with the הנאה, he is חייב. In our משנה the נהנה is מגלה דעת דניחא ליה through building an adjacent wall.
Alternately our משנה is considered a חסר because the מהנה raised the wall on account of the נהנה, that the נהנה should not cause him any היזק ראיה.
ובא בזמנו ואמר פרעתיך תוך זמני אינו נאמן – And the מלוה came on the date it was due, and the לוה said I paid you during the time before it was due, he is not believed.
We cannot infer from the דין of ר"ל, whether he maintains מיגו במקום חזקה or not; and similarly whether he maintains המלוה את חבירו בעדים צריך לפורעו בעדים or not. The case of ר"ל may be in a situation where there are עדים that the לוה owed the מלוה money and that the מלוה was קובע זמן. There is also no מיגו of פרעתיך היום, for that is a מיגו דהעזה.