A two story house -- of which the ground floor belonged to one person ("R") and the upstairs to another person ("S") -- fell down. S wanted to rebuild and R did not. S has a right to rebuild the ground floor alone and live in it until R pays him his expenses. However, the ground floor actually belongs to R. The Gemara says that although the downstairs belongs to R, and S who is living there is a "Gavra d'Avid l'Megar," still he does not have to pay rent for the time he lives in R's house. Seemingly, we see that "Zeh Neheneh v'Zeh Lo Chaser" is Patur from paying. The Gemara answers that this case is different, since the downstairs is obligated to the upstairs and therefore S actually has a right to live there for free.

D. Zupnik