We have learned that one violates the prohibitions of bal yera’eh and bal yimatzei only by possessing ĥametz that is the property of a Jew, as it is written, “no ĥametz of yours shall be seen” (Shemot 13:7). Yet, at first glance, there is a difficulty here, for it is written, “there shall be no se’or found in your homes” (ibid. 12:19), implying that it is forbidden to have ĥametz in a Jewish home under any circumstances. The Sages explained that, indeed, if the ĥametz belongs to a gentile, and the Jew has not undertaken the responsibility of its safekeeping and guaranteeing its return, then it is not forbidden to have it in the Jew’s domain, as it is written, “and no ĥametz of yours shall be seen.” If, however, the Jew accepted responsibility for the item, then it is considered like his own, and the prohibition applies. This is what the Torah intended when commanding: “there shall be no se’or found in your homes” (Pesaĥim 5b).
Therefore, if a Jew guaranteed the return of ĥametz that was deposited with him, it becomes like his, and he is not allowed to keep it in his home or courtyard, but must return it to the gentile or clear it out. In a situation where he cannot return it to the gentile and clearing it out will cause him a loss, he should sell the ĥametz together with the place it is stored, to a different gentile (SA 440:1; MB ad loc. 4). However, if the Jew undertook the protection of a gentile’s ĥametz that remains in the gentile’s possession, then the Jew does not violate any prohibition (MB 440:7). Thus an insurance company owned by a Jew may insure gentiles’ ĥametz, because it remains in their possession (She’arim Metzuyanim Be-halakha 114:29).
Furthermore, if a Jew deposited ĥametz with a fellow Jew for safekeeping, each of them is under obligation to clear it out – the depositor, since he owns the ĥametz, and the recipient of the deposit, since by undertaking to safeguard it he becomes like an owner (SA 440:4). Even if he did not undertake its safekeeping, he is required to clear it out.
If one bought stock in a company that owns ĥametz, and Pesaĥ arrived, if he has the authority to express his view about how to manage the company’s affairs – what to sell and what to buy – then it is considered as if he owns the ĥametz, and he violates bal yera’eh and bal yimatzei on account of it. But if he is not authorized to express an opinion, then he is like everyone who invests in stocks – the company owes him a percentage of its value, but its property is not considered his, and he does not violate any prohibitions on account of its ĥametz holdings. Accordingly, those who invest money in mutual funds or pension funds do not violate any ĥametz prohibitions, even though the managers of the funds may invest part of the money in a company that owns ĥametz, since this ĥametz is not considered the property of the investor (She’arim Metzuyanim Be-halakha 114:28).