This Halacha is an excerpt from our Sefer
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Owning stocks in companies who own/sell Chametz:
Some Poskim rule that owning stocks in a company that owns/sells Chametz, is not considered ownership of the Chametz of the company and hence one is not required to sell his stocks before Pesach. Other Poskim, however, rule that owning stocks of a company makes one be considered a Halachic partner of ownership in all their assets, including their Chametz. Accordingly, one must sell all his stocks in a company who owns/sells Chametz. Practically, the custom is to include one’s stocks in the Mechiras Chametz sale that is done before Pesach.
Wives: It is best for wives to appoint their husbands to include them in the Mechiras Chametz sale, by having him add their name to the Shtar Harsha. [See Chapter 5 Halacha 1 in Q&A!]
Money invested in a bank, hedge fund, investment firm:
One who has money in a bank, or firm, who invests their client’s money in Chametz owning companies is not considered an owner of the Chametz owned by the companies that the bank/firm have invested in.
 See Piskeiy Teshuvos 440:1
 Cheishev Haeifod 1:82 in name of Tchebiner Rav based on Mahariy Levi 2:124
 Minchas Yitzchak 3:1; 7:26; Moadim Uzmanim 3:269
 Heard from Harav Yaakov Yosef za”l
The reason: As all businesses today separate money from one’s salary which goes to a pension fund, and the money of that fund may be used to purchase Chametz stocks, in one’s name. Now, since according to law the husband has no access to that money, and even in case of divorce, he may not be able to have access to her funds, therefore it is considered under her ownership, and must be sold by her. Certainly, if she began working, and receiving a pension fund, prior to the marriage, then she retains the legal rights over it.
 Minchas Yitzchak 3:1; Piskeiy Teshuvos 440:1
 The reason for this is because the money given to the bank or firm is considered a loan, and hence it is the bank/firm who have purchased stocks or partnership into the invested companies, and not the original person who gave them the money. [Minchas Yitzchak ibid]