If אנסים took possession of the field after the לוקח made a חזקה, he cannot nullify the sale even if he bought the field באחריות. He can nullify the sale only when there are עוררים who claim the field is theirs legitimately.
משהחזיק בה אין יכול לחזור –
Once he took possession of it, he cannot retract
If a property is purchased with any קנין (besides קנין כסף), the buyer has the right to nullify the sale if יצאו עליה עוררים as long as he was not דייש אמצרי. And if he purchased it באחריות then even if דייש אמצרי it can be nullified.
When the buyer must pay, he can claim שקול ארעך בזוזך (by יצאו עליו עוררים), if the value of the property remains the same as at the time of the sale.
רב הונא said either כסף or רב הונא אמר או כסף או מיטב - מיטב
רב הונא statement is either in reference to the previous discussion of כסף and מיטב; or it is referencing our משנה.
The רי"ף maintains that one may initially pay שוה כסף for נזיקין.
The ר"ת (disagrees and) divides the payment obligation into three categories:
A. For נזיקין you may initially pay with either כסף or (even) מיטב; if not available then (only) also with שוה כסף. B. For בע"ח and a לוקח (in a מקח טעות scenario) payment must be with כסף, not מיטב (and certainly not שוה כסף); if כסף is not available, also (even) with שוה כסף (and certainly with מיטב). C. For a worker only with כסף; period!
פשיטא האי ברא והאי לאו ברא –
It is obvious! This one is a son and this one is not a son?!
When the sons both inherited property, then whosever property is taken away by their father’s creditor, suffers the loss. However where one son inherited land and the other money, the landowner must be compensated by his brother, if the land is confiscated by their father’s creditor.
ורב אמר בטלה מחלוקת – And רב says that the division is nullified
רב maintains that since the brothers are יורשים the חלוקה becomes בטל and it must be divided anew; while רב אסי maintains that while the disinherited brother must be compensated, the division remains. However if the מלוה is still in possession of the field it can be forcibly redeemed from him.
If we were to maintain that האחים שחלקו כלקוחות באחריות דמי, then if a בע"ח collected his debt from one of the brothers the other brother would only be required to compensate with money. The division however would remain intact.