(a) The gemara( 6 lines from bottom of B"M 101a) asks what is the consequence of retracting (asking for the wood and stones back) after imposing a benefit on another(building up a ruin/churvah). Rav nachman holds that he is allowed. My question is that on gittin 11b the mishna says that one can't disadvantage a person without their consent. In both gemara's it seems that throughout the story the recipient (the owner of the ruin and the woman/slave who is being divorced/freed) had no knowledge of the action of the other and yet the recipient made some kinyan on the benefit (kinyan chetzer (for the ruin), through the shaliach) and therefore the benefactor is no longer able to retract because retracting would cause a disadvantage to them. Could you explain rav nachman, or perhaps my assumption that there is a kinyan in the case of the builder is incorrect.

(b) Another question. The gemara earlier(15 lines from the bottom) on 101a seems to be very similar to the cases of "Zeh neheneh ve'Zeh chaser" from bava kama 20a-b. over there the gemara ruled that one is chayiv to pay. One example on B"K20b is one who fences in his friend's field. Thus, it seems that "Zeh neheneh ve'Zeh chaser" applies if a person took benefit for himself or imposed benefit on another. Then, all Rav says is that one must pay for a person who planted trees in your yard without your consent. Yet, if it's true that one must pay in such a case then what is statement of rav teaching me? Alternatively, why didn't the gemara b"k, when searching for a source, go to rav for his opinion here who seemingly addresses the issue. I hope the questions are clear. Thank you very much.

ike sultan, west orange, US