Beraisa: If one rents his house to his friend, before the tenant entered, the owner is obligated to destroy the Chametz; once the tenant has entered, the tenant is obligated to destroy it (as the Chametz is considered his property). R' Shimon said that this applies from the time that the tenant was given the key; before this time, however, the tenant is not obligated to destroy the Chametz.

Query (R' Yuda bar Pazi): If he gave over the key (as a deposit rather than as an act of rental), what is the law?

Answer (R' Zecharia): It is a dispute between R' Shimon and the Chachamim, as the Mishnah teaches -

If one gave over his house-key to an ignoramus, the house remains tahor, as he only gave him the key for safe keeping.

(Beraisa) R' Shimon says that the house is Tamei (by giving him the key, he has given over the entire house to the ignoramus).

(Beraisa): If a gentile comes to a Jew with Chametz in his hand, the Jew need not destroy it. If the gentile deposited it with the Jew, the Jew must destroy it. If the Jew designated a house for the gentile, he need not destroy the Chametz.

Query: If the Jew did not destroy the Chametz that had been deposited with him, what is the law?

Answer: R' Yona said that the Chametz is permitted and R' Yosa said that it is prohibited.

Explanation (R' Yosa): Even though it is the Chametz of a gentile, since the Jew transgressed by not destroying it, it becomes prohibited.