labochur
the case of the gemarah someone threw a cli from a roof and someone else broke it BK17b
we say potur in this case because of manah tvirah tavar but how to learn this and who will be potur is where the machlokes the ramban and rashi starts
ramban says [ ramban bsof perek keitzad haregel bmilchemes hashem] because ein lo damim– has no value once the chafetz is in the air and potur is going on both of them the first guy is potur since he wasn’t mazik [hezek she’aino nikar ] and the second guy broke something worthless and manah tvirah tavar so he is potur. so basar tavar maneh — not broken until the end of the zrikah.
Rashi learns potur is on the guy who breaks because basar m’ikarah the guy on the roof already broke it since with out the second guy it will break. hte second guy manah tvirah tavar — it already broke and we go mikarah
the famous question on this sugya is lfi haramban if you throw somethjing off the roof and nobody breaks it what is the din