Commentary for Pesachim 7:10

Giving over the Key – It is the acquisition of the rental. And if when he [the landlord] had not yet given over the keys the 14th began, the obligation to search fell on the landlord, because it was still in his domain, and even though he [the landlord] gave over [the keys] to him [the tenant] in the day, the obligation to search had already fallen on him [the landlord].

If From When the Key is Given Over - The explanation of Rashi is that transfer of the keys acquires. And this didn’t seem so to Ri [Rav Yitzchak ben Shmuel, a great-grandson of Rashi and student of Rabbeinu Tam] because it says in Merubah [7th Perek of Bava Kama] (BK 79a), “just as land is acquired by money, contract, and use, so rentership is acquired by money, etc.” And it doesn’t say that transfer of the key works except that it’s as though it [transfer of the key] says to you “use it and acquire it”, as it says in HaParah (BK 52a). And Ri explained that here, it is when he transfers to him the key and he hasn’t taken possession, and the one who has the key in his hand when the 14th falls is obligated to search, because the one who does not have in his hand the key, how can he enter and search? However, if a person entrusted the key to his house to another person, the person entrusted (Nifkad) is not obligated to search, but only if he wants to take possession of the house and acquire it.